refactor SourceCodeRepo heuristic

[art1f1c3R]

Referencing the TODO item in #983:

Currently, the SourceCodeRepo heuristic still has a dependency on the EmptyProjectLinks heuristic. This is done currently to make the merging process easier, and to avoid a heavy rework of the SUSPICIOUS_COMBOS dictionary. This dictionary is expected to soon be replaced with a more usable alternative, at which time the combinations including this heuristic may be updated.

The SourceCodeRepo heuristic must be refactored to avoid a dependency on the EmptyProjectLinks, as these two heuristics are not closely related enough for this dependency. SourceCodeRepo tracks whether a repository was found, whereas EmptyProjectLinks tracks whether any project links are available on the PyPI page of the package.