This fixes a bug where precompiled files were always being added to the default outputs
of a target. The intent is they are only added to a target's default outputs if the
target explicitly opted into precompiling. This went unnoticed because the exec tools
toolchain is still disabled by default, so the test that verifies a basic py_binary's
default outputs didn't generate implicit pyc files. It was introduced when fixing the
pyc collection bug.

To fix, only add the pyc files deemed "required", not all the pyc files generated.

Also added a test to capture this case to the precompile tests.