Generalize processors

[emdoyle]

This PR builds directly on #586

While that PR introduced the concepts of both the FileProcessor and FileChecker, it was primarily concerned with the FileChecker, composing logic from multiple checkers to produce a final stream of diagnostics.

This PR is focused on FileProcessor, and more specifically with generalizing the FileModule intermediate representation which they produce. This means:

• Cleanly separating ignore directive parsing from import parsing (remove NormalizedImports combination object)
• Creating the Dependency enum which can hold imports as well as other types of source code references
• Consolidating the FileModule type to be used across both pipelines, removing marker types
• Pushing low-level import parsing details into specific processors rather than in the high-level pipeline