This allows including additional applications or third party
plugins when creating a release, running the broker locally,
or just building from the top-level Makefile.

To include Looking Glass in a release, for example:

$ make package-generic-unix ADDITIONAL_PLUGINS="looking_glass"

A Docker image can then be built using this release and will
contain Looking Glass:

$ make docker-image

Beware macOS users! Applications such as Looking Glass include
NIFs. NIFs must be compiled in the right environment. If you
are building a Docker image then make sure to build the NIF
on Linux! In the two steps above, this corresponds to Step 1.

To run the broker with Looking Glass available:

$ make run-broker ADDITIONAL_PLUGINS="looking_glass"

This commit also moves Looking Glass dependency information
into rabbitmq-components.mk so it is available at all times.

In particular, the temporary `licensing` directory was never cleaned up.

Like other packaging files, they should not be part of RabbitMQ itself.
One day, they will probably be moved to a dedicated repository, like
other packaging files.

More precisely, it will leave the deps dir intact

Once the monorepo is built, from within it one can run `make
fetch-topic-branch-${TOPIC_BRANCH}` then `make
topic-branch-${TOPIC_BRANCH}` to rebase the commits from all the
sources back onto the monorepo

Add GitHub Actions workflows for Erlang/OTP 22.3 & 23.0.

The workflows run tests for each component that is now part of this
repo, with test suite parallelization specifically for the rabbit
erlang application.

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

When making `docker-image`

Ubuntu 18 fails to build the image unless tar is invoked with the capital J

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>