feat: add relative submodule URL resolution to rattler_git

[Hofer-Julian]

Description

As discussed in prefix-dev/rattler-build#2401, we want all git related operations to be bundled in rattler_git. So I move the submodule resolving here.

Originally submodules with relative paths failed since we first check them out and then git clone --local which hard-links and keeps the original relative path intact. That doesn't result into the paths we want, so we make them absolute with the original path as reference

How Has This Been Tested?

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  • I have tested any AI-generated content in my PR.
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Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added sufficient tests to cover my changes.