This patch teaches Zapp how to read Python toolchains to get an
interpreter, rather than hard-coding "python3" somewhat opaquely. This
enables the use of hermetic or otherwise selected interpreters.
Fixes#2
This patch teaches Zapp! to introspect the `sources` of a manifest, and
look for the well-known `WHEEL` file(s) indicative of an
unzipped/installed wheel in the input sources. A wheel can be (somewhat*)
correctly reassembled by zipping its unzipped state, so in the presence
of unzipped wheels Zapp! will re-zip them and enter them into the
manifest appropriately for inclusion.
This fixes#6 the nasty way, as there's no good way to make
`rules_python` provide wheel dependencies or to translate unrolled
wheels back to wheels during rule execution as this would violate
Bazel's file dependency model.
