Fix `_legalize_path` types

[snejus]

Description

Part 2 of the work fixing types in beets.util.__init__ #5215.

Mypy was not happy here because _legalize_stage function implementation concatenates path and extension parameters, implying that their types need to match.

You can see that initially path parameter was defined as a str while extension was bytes.

In reality, depending on the fragment parameter value, extension was sometimes provided as a str and sometimes as bytes. The same parameter decided whether path gets converted into bytes within _legalize_stage implementation. No surprise that mypy was confused here.

_legalize_stage is only used within Item.destination method implementation which is where fragment is defined. I determined that the fragment parameter controls the form of the output path:

• fragment=False returned absolute path as bytes (default)
• fragment=True returned path relative to the library directory as str.

Given the above, the change

1. Renames fragment parameter to relative_to_libdir for clarity

2. Makes Item.destination to return the same type in both cases. I picked bytes since that's the type that majority of the code using this method expects.

   I converted the output path to str for the code that has been expecting a string there.

3. Decouples _legalize_stage and _legalize_path implementations from the relative_to_libdir. The logic now uses str type only.