Pass root path to FullyQualifiedNameProvider

[amyreese]

Summary

This allows FullyQualifiedNameProvider to work with absolute paths,
rather than assuming all paths given will be relative to the current
directory. This enables tools like Fixit to provide a root path, and
have the FullyQualifiedNameProvider correctly scope the final results
relative to that root path.

This does require that both the root path and the given file paths
match the other as relative or absolute, due to the
calculate_module_and_package helper comparing file paths relative
to the root path, but this seems like a reasonable tradeoff, and
unlikely to cause a problem in normal use cases.

Test Plan

Updated test cases. Works with in-progress Fixit2 changes.

[codecov-commenter]

Codecov Report

Base: 94.79% // Head: 94.79% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (73e6daa) compared to base (8aebbb6).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #867   +/-   ##
=======================================
  Coverage   94.79%   94.79%           
=======================================
  Files         249      249           
  Lines       25831    25835    +4     
=======================================
+ Hits        24487    24491    +4     
  Misses       1344     1344           

Impacted Files	Coverage Δ
libcst/_types.py	85.71% <100.00%> (+5.71%)	⬆️
libcst/helpers/module.py	100.00% <100.00%> (ø)
libcst/metadata/name_provider.py	98.64% <100.00%> (ø)
libcst/metadata/tests/test_name_provider.py	100.00% <100.00%> (ø)

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[davidshepherd7]

FWIW I also ran into this bug and tried the same fix (passing root_path instead of "." in gen_cache, and this fix allowed me to use FullyQualifiedNameProvider as a METADATA_DEPENDENCIES in a codemod where before it was broken.

I was just about to start writing a PR for the same thing when I saw this one! 🎉 😍

Also: fixes #722 (someone else there had the same issue and the same solution).