Allow/Ignore specific manifests

Wanted to bring up this issue again: https://github.com/dependabot/dependabot-core/issues/1629 which got closed due to no response.

Having an option to inspect specific files would make configuration a lot easier. In my case, we have multiple requirements.txt files all in the same directory, but we want to inspect just one of these requirements files.

Start by reading issue #1629 and tracing how Dependabot selects requirements.txt manifests when multiple files share a directory. No implementation files or tests are named in this issue; done means configuration can target only the specified manifest while preserving normal dependency inspection behavior.

Written by the indexing model from the issue text.