This changes templates so they return an entire context dict instead of
only returning context based on their name. For example, now the 'path'
template can set context for 'env'.
A side effect of this is that there is no longer a 1-to-1 mapping of templates
in tryselect and taskgraph.
MozReview-Commit-ID: IHRXXi5mB4G
This allows subparsers more control over which sets of arguments they need to
implement. For example, it doesn't make sense for the 'empty' selector to
accept the preset arguments. Now it can opt-out of those and only implement
the 'push' arguments.
MozReview-Commit-ID: GOfjcFtlfDD
The main motivation behind this change is to make testing easier, so e.g:
./mach try fuzzy --no-push
and
./mach try syntax --no-push
both work the same way.
MozReview-Commit-ID: LmjA3Kq7xKN
