Proposal for incremental path refactor

[gsoltis]

This PR is a proposal for how we can start to get a handle on the various kinds of paths that turbo uses.

My goal is the following:

• Eventually ensure that an absolute, platform-dependent path is used any time we interact with the filesystem
  • This will allow us to guarantee that we don't break --cwd, which is likely currently broken
• Eventually ensure that we can easily identify relative paths, both with respect to the monorepo root and to a particular package root
• Be incrementally adoptable. There is a huge amount of surface area to this change, so it's important to be able to tackle it a piece at a time
• Be auditable. The goal would be to eventually ban usage of path/filepath and a subset of os.* functions anywhere in the codebase outside path.go. It may even be possible to automate this in CI. Furthermore, entry points and exit points from our custom types (currently just AbsolutePath) are clearly marked, along with the intention of eventually migrating them, if appropriate (e.g. ToStringDuringMigration, UnsafeToAbsolutePath).
• New PRs that interact with the filesystem should use fs.AbsolutePath, even if they need to use one of the migration functions to facilitate that. Hopefully this will force both the submitter and reviewer to focus on ensuring that the new code will work properly with --cwd.

I've done a few pieces of the migration to show what it would look like in the code base in a partially migrated state. path.go contains the AbsolutePath type, which ideally would be the sole way to interact with the filesystem. Please let me know what you think about the api and the migration overall.

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[jaredpalmer]

Should we call this SafeAbsolutePath?