This makes binary-level opt-in/opt-out of precompiling work as intended. Previously,
when pyc_collection=include_pyc was set on a binary, only transitive libraries that
had explicitly enabled precompiling were being included (which was moot anyways --
libraries put their files in runfiles, so no matter what, their files were included).

The intent was that, when a binary set pyc_collection=include_pyc, then precompiled
files would be used for all its transitive dependencies (unless they had, at the
target-level, disabled precompiling). Conversely, if pyc_collection=disabled was set,
the precompiled files would not be used (unless a target had, at the target level,
enabled precompiling).

To make it work as desired, the basic fix is to make it so that libraries have a place to
put the implicit pyc files (the ones automatically generated), and have the binaries
include those when requested. The net effect is a library has 4 sets of files it produces:

• required py files: py source files that should always go into the binary's runfiles
• required pyc files: precompiled pyc files that should always go into the binary's
  runfiles (e.g., when a library sets precompile=enabled directly).
• implicit pyc files: precompiled pyc files for a library that are always generated, but
  it's up to the binary if they go into the runfiles
• implicit pyc source files: the source py file for an implicit pyc file. When a binary
  doesn't include the implicit pyc file, it must include the source py file (otherwise
  none of the library's code ends up included).

Similarly, in order to allow a binary to decide what files are used, libraries must
stop putting the py/pyc files into runfiles themselves. While this is potentially
a breaking change, I found that, within Google, there was no reliance on this behavior,
so should be safe enough. That said, I added --add_srcs_to_runfiles to restore
the previous behavior to aid in transitioning.

BREAKING CHANGES

1. py_library no longer puts its srcs into runfiles directly.
2. Removed --precompile_add_to_runfiles
3. Removed --pyc_collection
4. precompile=if_generated_source removed
5. precompile_source_retention=omit_if_generated_source removed

Though 2 through 5 are technically breaking changes, I don't think precompiling
was very usable anyways, so usages of those flags/values is rare.

Fixes #2212