gftools-packager --build-from-source

[simoncozens]

This allows us to build a font from source while packaging it.

Notes:

• Currently it runs gftools-packager on the first config.y*ml or source file it finds in the sources directory. It does not run "make build"
• Because of this, it runs in the packager's venv. This is deliberate, allowing us to rebuild with a newer toolchain than the frozen one in the repo (bugfixes!). I started the plumbing for a their_venv flag, but haven't implemented it yet.
• You can provide a "local" config.yaml file in the googlefonts/ofl/familyname directory which is copied into the cloned repository's directory. This potentially allows us to use gftools-builder on things which are not set up to use it in the upstream, or to fiddle with the flags and settings.