Copies the installed dependency graph into the project vendor/ directory.

Usage:
  v vendor

Run after v install. Both direct and transitive dependencies are copied using
canonical import paths. All dependencies must be installed. No remote is queried.
The nearest v.mod selects the project root, including when run in a subdirectory.

Existing vendor directories and symlinks are preserved. Remove an existing vendor
explicitly before creating a fresh copy. A failed staging copy is not published.

To compile using the copy, set VMODULES to the project's absolute vendor path.
