For a long time two copies of the 'taskgraph' module have existed in parallel.
We've attempted to keep them in sync, but over time they have diverged and the
maintenance burden has increased.

In order to reduce this burden, we'd like to re-join the two code bases. The
canonical repo will be the one that lives outside of mozilla-central, and this
module will depend on it. Since they both have the same module name (taskgraph)
we need to rename the version in mozilla-central to avoid collisions.

Other consumers of 'taskgraph' (like mobile repos) have standardized on
'<project>_taskgraph' as their module names. So replicating that here as well.

Differential Revision: https://phabricator.services.mozilla.com/D127118