In order to be able to do offline builds for mobile we need to collect and
distribute Glean dependencies beforehand.[1]

There are three steps involved in the process:
  1) Finding out the `glean_parser` version
  2) Downloading and verifying the dependencies, and creating the tarball
  3) Pushing it to the sources mirror

1)

Running `make list_toolchain_updates` should tell you which version of
`glean_parser` is needed.

2)

Update `version` in `projects/glean-parser/config` to the version from step 1 above.

Run this command to generate the tarball:

  make create_glean_deps_tarball

There is no really good way (yet) to verify that `pip` gave us actually what we
wanted.
We can download `glean_parser` and its dependencies using a different
network path.
Comparing the results of both downloads should yield the exact same binaries.
Notice that pip installs the most recent version of a dependency by default, so
verification may fail if it is run sometime after the archive was created the
first time.

3)

Upload the compressed tarball to the sources mirror and update the SHA-256 sum of
it as well as the `glean_parser` version in the respective projects.

[1] See: https://bugzilla.mozilla.org/show_bug.cgi?id=1651662 for details.
[2] https://github.com/mozilla/glean
