rebar3 would crash if the option `{keep_logs, N}` was present and
there were more than N run directories in the log dir, so if the
option is found we keep the N-1 newest directories and delete the
rest.

While we are at it, as this should fix the crash, let's limit the
number of logs runs to keep from test runs in rebar3 to 5.

End result should allow to look like this:

    $ head -n 3 rebar3
    #!/usr/bin/env escript
    %% Rebar3 3.14.0-rc1
    %%! +sbtu +A1

This will allow tools like mix to read the top of the file to know the
version being used in the current path (when using features that aren't
forwards-compatible) without having to invoke the tool itself.

The version (full dynamic one) is also added to the local install:

    $ head -n 2 /home/ferd/.cache/rebar3/bin/rebar3
    #!/usr/bin/env sh
    ## Rebar3 3.14.0-rc1+build.4599.ref6b0f90d

The `##` comments are used to keep a fixed width over the other form.

To be backported to 3.13.2 as well.

This commit switches the relx dependency to branch 4.0.0. It is the
first iteration of integration with relx 4.0 which adds many changes
to how relx works and how the two integrate.

rebar_relx module now creates a relx state from the relx configuration
and pass it along with all the OTP applications rebar3 knows about
which may be used in the release. This allows relx to not have
to duplicate the effort of application discovery like before.

Since this is intended to be released only when OTP-23 is released
the testing of OTP-18 has been removed.

- Can't deal with := in map type declarations
- maps:update_with didn't exist back then so we reinvent it for the few
  months until we deprecate OTP 18

Fixes #2159; this is done by force-reloading the handlers to match the
config.
This can create a bit of a funny effect if sys.config specifies an INFO
log level (or lower) is specified. While apps are booted for config
changes before the cth_failonly hook is enabled, supervision and other
application log messages can start being output. They will start being
suppressed once the CT run begins.

This is not a bug, it's a race in instantiation of control and enabling
of log levels. Nothing we can do about that. It might however surprise
people a good bit. If non-default stdout handlers are added, they are
similarly likely to become noisy; specifying a test-specific sys.config
file may be necessary then.

Also includes a bump of cth_readable version, which now checks for
updates to the log formatter on every test output.

- fix some bad typespec
- bump the meck version for good fortune

fixed #2063

- vendor in hex_core at v0.5.0
  - Change where repo_name should be the org and not the parent
  - Changed rebar_utils:url_append_path/2 to not explicitly add a '?',
    this is returned in the Query chunk by http_uri:parse/1 (e.g., "?foo=bar")
  - update organization_merging test to expect the sub-repo as the repo_name
  - Add tests for rebar_utils:url_append_path/2
  - Stop referencing/setting "organization" in config and use new
     organization settings (api_repository and repo_organization)
  - Do not set (assume) the read key is valid for any/every repo
  - Set repo_organization and api_repository to org
  - Update tests to check for new config opts

Handles some internal changes in logger in OTP-21.2 that broke the
follow-up error printouts after the first failure of a CT run

* 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