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.

* 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

* 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

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

Bump cth_readable for OTP-21 compat

Revert "upgrade certifi to 2.3.1"

upgrade relx to 3.25.0

upgrade certifi to 2.3.1

upgrade bbmustache to 1.5.0

Finish adding specs for private funs in rebar3.erl

- added type specs for following private functions:
    - run/1
    - set_options/2
    - test_state/1
    - safe_define_test_macro/1
    - test_defined/1

run compile provider in default namespace from bare

Resolve string vs list crashing rebar3

- resolves #1645

Create ERLANG_LIB_*_erl_interface environment variables only if erl_interface exists

Introduce support of add and del operations under erl_opts directive

Add hooks to the upgrade command

Add --start-clean flag to rebar3 shell

Allows override of apps specified in other on-disk options. Equivalent
to `--apps=""`

Fixes #1785

As requested in #1733