Stricter path types for stricter path-related logic

[fsoikin]

Description of the change

Summary of prior discussion:

• Paths in Spago are handled in a bit of an ad-hoc way. To name a few examples: some places use local/partial paths and rely on current directory being the right one; other places construct paths out of pieces; the cwd value is unsafePerformEffected at init and then ends up baked into various values around the code.
• This would be a problem for implementing spago script.
• This would be a problem for implementing In monorepos, allow running commands from package directories #1237
• We agreed to introduce stricter types to represent paths in Spago code.

In this pull request:

• New module Spago.Path
• Three new types for logically different kinds of paths - RootPath, LocalPath, and GlobalPath. See comments on the types for explanation.
• All lower-level utilities, such as Spago.FS, take strongly typed paths as parameters.
• This creates ripple effects, forcing the whole codebase to use strongly typed paths.
• Most command environments now have a rootPath :: RootPath and use that path as root for all operations, no longer relying on CWD for this purpose.
  • Some commands, such as registry, don't need a root path.
• Halogen apps are not using the new types. I'm not yet familiar enough with them to make the changes.

Checklist:

• Added the change to the "Unreleased" section of the changelog
• [ ] Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

[fsoikin]

Alright, I finally figured out the Windows failure, and it turned out to be an actual potential problem, not just a silly forward vs. back slash issue. I added special handling for it and some tests.

[f-f]

Looks great now - awesome job 👏