Moved get_use, get_manifest, get_sources and get_default_sources definitions...

Moved get_use, get_manifest, get_sources and get_default_sources definitions to the base Pybuild class so they can be used on cached Pybuild objects.

Unfortunately this requires function-level imports, as so many files import portmod.pybuild due to the Pybuild class being used for typing.

Should allow !149 (merged) to use cached pybuilds to scan sources.