Better vendor experience

[albersonmiranda]

This PR:

1. Enables the user to opt in to cleaning the vendor directory after creating the tarball: vendor_pkgs(clean = TRUE). Defaults to FALSE.
2. Updates Makevars so it looks for src/vendor/ first. If not present, it will then unzip the tarball (useful for projects with heavy dependencies).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.20%. Comparing base (1688f4b) to head (ea9de03).

Additional details and impacted files

Files with missing lines	Coverage Δ
R/cran-compliance.R	81.81% <100.00%> (-2.60%)	⬇️

... and 1 file with indirect coverage changes

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