feat: clean up path handling

[Gankra]

• Depend on camino to avoid needless conversions of the camino paths cargo_metadata gives us
• Add a StoredPath(Buf) type that is for paths that will get written to output files like main.wxs
  • Most interesting part is StoredPathBuf::from_utf8_path
• Clean up code to use new path types more

[Gankra]

Currently a draft because I want to do some more testing that this approach is sane

[Gankra]

Experimentally turned on unit test CI for linux/macos, working on enabling more of the tests that make sense (it seems like the initialize::execution unit tests are messed up -- they use set_current_dir but weren't using serial_test, and even if they do every other test is racing them. No idea why this isn't a problem on windows...)

[Gankra]

@volks73 I'm now reasonably confident in the implementation, and have everything except for create execution tests being tested on all 3 platforms, which is the functionality one would expect to work cross-platform. Note that this change largely doesn't affect actual behaviour on windows, so it's mostly just trying to make other platforms behave more like windows (ostensibly desirable).

[Gankra]

(additional evidence: i pointed the cargo-dist integration PR to this branch and now the snapshots are the same on all platforms -- without me having to regenerate them, so the windows ones never changed)

https://github.com/axodotdev/cargo-dist/pull/370