Update FileFinder to return std::filesystem::path when handling full file path

[rosswhitfield]

Description of work

This changes the return type of file paths to use std::filesystem::path instead of string. I also tried to make the parameter names more consistent between different methods for things like the hint.

Note: On unix systems std::filesystem::path will implicitly cast to std::string. But on windows that doesn't happen so a lot of .string() was needed to be added, mainly for tests.

Part of work towards #40542

To test:

Code review is all.

This does not require release notes because internal change only not visible to users.

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[peterfpeterson]

Much more sensible code. Sorry that changing the return required changing a bunch of other places.