Simplify python local dist handling code.

[benjyw]

Previously, BuildLocalPythonDistributions generated local dists and put them
in products. Downstream tasks then synthesized PythonRequirementLibrary
targets to point to the local dists, if they needed to resolve them. This was clunky,
and required special knowledge of local dist issues to reside in multiple downstream
tasks.

This change has BuildLocalPythonDistributions itself inject the synthetic
PythonRequirementLibrary targets - one per dist - and stitches them into the
build graph. Downstream tasks no longer need to know or care about local dists
(apart from declaring them as required products), they just see PythonRequirementLibrary
targets, whether synthetic or organic, and handle them uniformly.

Note that previously PythonBinaryCreate had to do some footwork to ensure that only the
dists depended on by a specific PythonBinary were used when creating that binary.
This now happens naturally, since we generate one requirement library per dist,
and the dependencies are wired up in the right way.

[jsirois]

As noted in other discussions today in slack and in comments on #5479, this is broken since the requirement will be stable across source file edits (until version itself is edited in setup.py). That's a pre-existing problem though.

[benjyw]

I assume this will be fixed by creating unique versions, rather than directly special-casing in this or other code.

[CMLivingston]

LGTM! Thanks for making the improvement.