Support rebar config dir env var

Make missing file report more comprehensive

Warn when there's mismatch between rebar.lock & original package checksum

Gracious loading of unloaded but blacklisted mods

Only lint app files when compiling

try CIRRUS_TAG != '' for triggering cirrus for new tags

This allows to reduce the number of noise to only checking deps' app
files when they're built, rather than on every run.

Since main apps and checkouts are still compiled every time, the linting
takes place there and then with a higher frequency.

rebar3's shell allows people to set applications as blacklisted to
prevent them from being reloaded because that can cause crashes.
However, as part of its normal operations, rebar_paths unloads all
modules that are currently not "owned" by at least one process,
considering them safe to do so.

These two behaviours, put together, lead to an odd thing where some
modules are suddenly unloaded and not in path, and that can be
confusing.

This calls for a unification of both features. We could decide to be
pushing the complexity of rebar3's shell into rebar_path so it knows of
blacklists, but this would be a bad idea because rebar_agent already
owns all the damn hack.

So instead this fix adds an optional call within rebar_agent's
blacklisted applications handling that calls `code:ensure_loaded/1` on
their modules. This avoids forcing any code change that would cause a
crash, but reinstates unloaded paths that could be confusing.

Addresses some comments in #2013

bump relx to 3.32.1 and providers to 1.8.1

add support for usage message after template is done

fix install of s3cmd for publishing releases in cirrus

upgrade ssl_verify_fun to 1.1.5

bug when reread_config running on OTP 17

add support for reading in environment variables from a file

Co-Authored-By: Fred Hebert <mononcqc@ferd.ca>

upgrade relx to 3.32.0

add nightly and release upload tasks to cirrus and remove travis

Do not apply overrides to a root application.

Overrides should apply to a layer below where they are declared. This
patch makes it so if the project root is an application (i.e. it isn't
'root' and therefore not an umbrella project), we omit applying
overrides in rebar_app_discover.

This in turn required changing a bunch of tests, because all the tests
worked with the idea that all overrides applied to all apps to validate
that they get inherited properly. The changes re-structure the cases so
they are written with an umbrella app, demonstrating that the changes
stick.

OTP-22.0 readiness

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

add support for git ref and file content as app version

supervisor template: add missing comma

remove large comment blocks from templates and use maps in sup