PSA: Renaming `taskcluster/taskgraph` -> `taskcluster/gecko_taskgraph`

[Andrew Halberstadt]

tl;dr: Bug 1732723 will rename the taskcluster/taskgraph directory to taskcluster/gecko_taskgraph. Apologies in advance for the conflicts!

Context:

For many years releng has maintained two copies of the `taskgraph` module. One that lives in mozilla-central and used by Firefox, and another standalone taskgraph that is used by consumers outside of central (like Fenix, android-components, etc). We've tried to keep these two repos as in-sync as possible, but the maintenance burden has always been high and the goal was to eventually try and merge them back together.

To that end, the taskgraph in mozilla-central will start depending on the standalone one. Since they both currently use the same `taskgraph` module name, we need to change the name of the module in mozilla-central. The convention in mobile repos was already to use `<project>_taskgraph` as the name, so we're bringing Gecko in line with those.

Let me know if you have any questions,

Andrew

[Andrew Halberstadt]

This has now landed on central.