Remove python2 support from soong rules

Python2 has been removed from the android build.

Removing the python version transition mutator should lead to some
slight analysis time performance improvements.

I also made embedded_launcher in base_properties no longer
blueprint:"mutated", so that we can migrate usages to the main property
struct and remove the `version` property struct.

Many python tests were not being run due to a
`if d.desc != "module with duplicate runfile path" { continue }` line,
I removed that and fixed the tests. I also removed the test for an
error when there are no source files, because that check has been
relaxed since, and I think it's legitimate to have a python library that
only has data files, not source files. And there are some of those
libraries in the source tree.

Bug: 203436762
Test: m nothing --no-skip-soong-tests
Change-Id: Idf18e39233418237ecc83f70f65d83d4ea496633