Scan for cross-platform inconsistencies in locked requirements files

[ncoghlan]

venvstacks lays out the generated requirements files such that all of the platform-specific lock files for each environment are stored in the same directory.

This issue covers adding a helper script that scans a specified set of requirements subfolders looking for cases where the same requirement is present in multiple files, but with a differing version (some requirements being missing entirely is fine, as platform-specific dependencies are one of the main reasons platforms don't share lock files).