feat(manager/pip-compile): Handle some edge-cases with -r dependencies

[mbudnek]

Changes

This adds handling for two edge-cases I missed in #28052:

1. A -r dependency on a multi-input lock file; i.e.

graph LR
common.txt[[common.txt]]
final.txt[[final.txt]]
1.in --> common.txt
2.in --> common.txt
common.txt --> 3.in
3.in --> final.txt

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2. An input file with multiple transitive -r dependencies on the same file, i.e.

graph LR
common.in --> mid1.in
common.in --> mid2.in
mid1.in --> final.in
mid2.in --> final.in

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CC @not7cd

Context

#28050
#28052 (specifically #28052 (comment))

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 37.301.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 37.301.0

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