Move path management out of rebar_utils manual code path function
handling (which we leave there for backwards compat), and centralize
them to allow easier coordination of paths between plugins and deps.

On top of path handling, do a check of loaded modules to only purge and
reload those that actually need it done in order to prevent all kinds of
weird interaction and accidental purge kills. It also allows the
possible cohabitation of both at once, with a "in case of conflict pick
X" as a policy

Changing path handling in providers also highlighted a bunch of bugs in
some tests and appears to fix some in other providers, specifically
around plugins.

Re-add erlc compiler; plugins need it

However, we add a once-printed deprecation warning for the new
interface.

* bump bbmustache to version 1.6.0

* Use alternative bbmustache brackets in templates

This fixes the problems where literal nested tuples (`{{a,b},c}` or
`{{a,b}}`) would blow up and spaces needed to be inserted.

* add compile type for dynamic project compilation

* new rebar_compiler abstraction for running multiple compilers

rebar_compiler is a new behaviour that a plugin can implement to
be called on any ues of the compile provider to compile source
files and keep track of their dependencies.

* fix check that modules in .app modules list are from src_dirs

* use project_type to find module for building projects

* allow plugins to add project builders and compilers

retired packages are now used the same as any other but
a warning will be printed when it is resolved.

prerelease versions are skipped unless explicitly given
as the version in the constraint or lock file.

Fix sample config punctuation.

When REBAR_CONFIG was set it would not effect the top level app's
configuration because app_discover was rereading the top level
rebar.config which ignored REBAR_CONFIG. Instead this patch has
it use the existing configuration from REBAR_CONFIG.

top level default profile plugins are installed in the run_aux
function. This commit removes the plugins from the plugin opts
of an application that is also at the top level and skips the
default profile when installing top level plugins in
project_apps_install so they aren't handled twice.

Additionally, before handling a plugin the state's list of
known plugin apps is checked and the plugin is skipped if it
has already been handled.

* update to hex_core for hex-v2 repo support (#1865)

* update to hex_core for hex-v2 repo support

This patch adds only single repo hex-v2 support through hex_core.
Packages no longer filtered out by buildtool metadata and the
package index is updated per-package instead of fetched as one
large ets dump.

* tell travis to also build hex_core branch

* support list of repos for hex packages (#1866)

* support list of repos for hex packages

repos are defined under the hex key in rebar configs. They can be
defined at the top level of a project or globally, but not in
profiles and the repos configured in dependencies are also ignored.

Searching for packages involves first checking for a match in the
local repo index cache, in the order repos are defined. If not found
each repo is checked through the hex api for any known versions of
the package and the first repo with a version that fits the constraint
is used.

* add {repos, replace, []} for overriding the global & default repos

* add hex auth handling for repos (#1874)

auth token are kept in a hex.config file that is modified by the
rebar3 hex plugin.

Repo names that have a : separating a parent and child are considered
organizations. The parent repo's auth will be included with the child.
So an organization named hexpm:rebar3_test will include any hexpm
auth tokens found in the rebar3_test organization's configuration.

* move packages to top level of of hexpm cache dir (#1876)

* move packages to top level of of hexpm cache dir

* append organization name to parent's repo_url when parsing repos

* only eval config scripts and apply overrides once per app (#1879)

* only eval config scripts and apply overrides once per app

* move new resource behaviour to rebar_resource_v2 and keep v1

* cleanup use of rebar_resource module and unused functions

* cleanup error messages and unused code

* when discovering apps support mix packages as unbuilt apps (#1882)

* use hex_core tarball unpacking support in pkg resource (#1883)

* use hex_core tarball unpacking support in pkg resource

* ignore etag if package doesn't exist and delete if checksum fails

* add back tests for bad package checksums

* improve bad registry checksum error message

Reload logger config in shell

This requires some fancy dynamic work since the logger is started as
part of the kernel and we lost the sys.config from users when working
from there.

We start conservatively by making it an optional thing, turning it on
only where we know it to be safe.

The changes are applied _after_ having loaded the rest of configs so if
an inoffensive error happens, the shell works (with a bad error message)
rather than plain exploding.

add report in the effects_code_generation, as it is the short form for both report_errors and report_warnings

Friendlier output on include_lib errors

Turns output like this:

    ===> Compiling apps/c/src/c.erl failed
    apps/c/src/c.erl:3: can't find include lib "parse_trans/include/codegen.hrl"

Into:

    ===> Compiling apps/c/src/c.erl failed
    apps/c/src/c.erl:3: can't find include lib
    "parse_trans/include/codegen.hrl"; Make sure parse_trans is in your app
    file's 'applications' list

Which is likely going to help newcomers encountering issues.

Bump certifi

fixed typo erl_first_files

Includes an update to parse_trans on an OTP-21 friendly version

smarter shell code reloading, add env options for refresh_paths & refresh_paths_blacklist

Warn but succeed when atoms are passed to erl_first_files

otp21 eperm fix for local install/upgrade, no need to set uid/gid

otp-21 eperm fix: bump of erlware_commons and relx

- Introduce support of add and del operations for overrides
 - OTP-21 compatibility
 - Bump cth_readable for OTP-21 compat
 - upgrade relx to 3.25.0
 - upgrade bbmustache to 1.5.0
 - run compile provider in default namespace from bare compiling (fixes
   hooks for mix builds)
 - Resolve string vs list crashing rebar3 in erl_first_files
 - Create ERLANG_LIB_*_erl_interface environment variables only if erl_interface exists
 - Add hooks to the upgrade command
 - Add --start-clean flag to rebar3 shell
 - Auto-boot main app in OTP app project templates
 - Use maps for child spec examples in templates
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Merge tag '3.6.0' into refresh_paths

Bump to 3.6.0

- Introduce support of add and del operations for overrides
- OTP-21 compatibility
- Bump cth_readable for OTP-21 compat
- upgrade relx to 3.25.0
- upgrade bbmustache to 1.5.0
- run compile provider in default namespace from bare compiling (fixes
  hooks for mix builds)
- Resolve string vs list crashing rebar3 in erl_first_files
- Create ERLANG_LIB_*_erl_interface environment variables only if erl_interface exists
- Add hooks to the upgrade command
- Add --start-clean flag to rebar3 shell
- Auto-boot main app in OTP app project templates
- Use maps for child spec examples in templates

Add OTP-21.0 to Travis