It is desirable for e.g. smooth toolchain transitions, to be able to
refer them with generic name from toolchain jobs, while they have more
specific names, including version numbers. For example, in a near
future, there could be a linux64-gcc-4.9 toolchain and a linux64-gcc-6.
The default would be former, but at some point we'd want to switch to
the latter, without having to change all the toolchain definitions.
Moreover, when the switch happens, it would be desirable to have some
jobs stick with the old version, which is hard to keep track of when
all the toolchain definitions for build jobs use the same versioned
toolchain. With an alias, jobs that want the default use the alias, and
jobs that want to use a specific version use the versioned toolchain
name.
Such a definition automatically sets up the corresponding dependencies
in the taskgraph, and adds the necessary artifact definitions for use in
the corresponding jobs. The jobs end up with a MOZ_TOOLCHAINS
environment variable with a list of path@task-id strings, where task-id
is corresponding to the (possibly optimized) toolchain job, and path
corresponding to the toolchain-artifact defined for that toolchain job.
