Support building distributions from local patches

[terrykong]

When specifying the patches to build a distribution, you currently can only specify a single git-ref for a patch. This is an issue for reproducibility since if someone updates the commit the git-ref is pointing to, you can no longer build a previous build. If we support local patches, then we should be able to reproducibly build a distribution.

One proposal for the change would be in addition to the already supported ref specs:

pull/$number/head
mirror/$branch_name
mirror/pull/$number/head

We can support

file://.github/container/patches/$library/${feature_name}.patch