fix: load target_platforms through the hub (#2781)

This PR moves the parsing of `Requires-Dist` to the loading phase
within the `whl_library_targets_from_requires` macro. The original
`whl_library_targets` macro has been left unchanged so that I don't have
to reinvent the unit tests - it is well covered under tests.

Before this PR we had to wire the `target_platforms` via the
`experimental_target_platforms` attr in the `whl_library`, which means
that whenever this would change (e.g. the minor Python version changes),
the wheel would be re-extracted even though the final result may be the
same.

This refactor uncovered that the dependency graph creation was incorrect
if we had multiple target Python versions due to various heuristics that
this had. In hindsight I had them to make the generated `BUILD.bazel`
files more readable when the unit test coverage was not great. Now this
is unnecessary and since everything is happening in Starlark I thought
that having a simpler algorithm that does the right thing always is the
best way.

This also cleans up the code by removing left over TODO notes or code
that no longer make sense.

Work towards #260, #2319