internal utilities to find rust crate and cargo manifest

[kbvernon]

This PR adds internal utility functions that find R package root, build path from there to crate directory or Cargo.toml, and check that directory or file exists. Both return paths. These can be used to consolidate code and provide stronger checks.

Note: this is to simplify efforts to refactor functions to execute Cargo subcommands using processx::run() in PR #397.

[JosiahParry]

Looks good! we need to just ensure that the documentation doesn't actually get rendered. That will lead to a rejected package in CRAN

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.41%. Comparing base (4aefce1) to head (2de41fc).
Report is 1 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
R/find_extendr.R	100.00% <100.00%> (ø)

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