lib.rs has grown quite large, and mixes two responsibilities:
1. executing patch operations
2. parsing apply_patch invocations via a shell command
This PR splits out (2) into its own file, so we can work with it more
easily. We are explicitly NOT moving tests in this PR, to ensure
behavior stays the same and we can avoid losing coverage via merge
conflicts. Tests are moved in a subsequent PR.
---
[//]: # (BEGIN SAPLING FOOTER)
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* #8111
* __->__ #8110
