Generalize checks

[emdoyle]

This PR introduces the concept of a 'DiagnosticPipeline' to characterize the behavior of tach check and tach check-external. These Pipelines can be constructed by implementing a 'FileProcessor' to extract relevant information from incoming files, and a 'FileChecker' to produce Diagnostics from the processed information. These changes serve several purposes:

• Clearly separate concerns between parsing source files and producing diagnostics, and further separate concerns between dependency checks, interface checks, and ignore directives
• Reuse some shared concepts between check and check-external (e.g. ignore directives, eventually modules)
• Prepare for the introduction of more complex file processing (e.g. detecting framework magic like Django FKey string references, related models)
• Make space for solutions to option to report unused dependencies in tach check-external as an error instead of a warning #524 Enable to explicitly disallow import of certain external dependencies #535 FR: add support for dependency-groups #565 (generally, enhancements to check-external will be able to easily interact with the Tach module system)