Python: Limit `RequestInputAccess` to immediate uses
Python: Limit `RequestInputAccess` to immediate uses

This fixes some spurious results that occurred when we considered
_any_ use of `request.something` to be a source, even ones we had
tracked into other functions. To prevent this, using
`getAnImmediateUse` better captures the fact that we want the source
to be just the actual attribute access.

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