Inaccurate warning for ignoring vendored packages with subpath and version

[ron-shinall]

I noticed a warning that a couple of my vendored packages were being ignored as though there were unversioned:

pin "pdfjs-dist/build/pdf.min.mjs", to: "pdfjs-dist--build--pdf.min.mjs.js" # @5.4.530
pin "pdfjs-dist/build/pdf.worker.min.mjs", to: "pdfjs-dist--build--pdf.worker.min.mjs.js" # @5.4.530

For example, when executing bin/importmap audit:

$ bin/importmap audit
Ignoring pdfjs-dist/build/pdf.min.mjs (vendor/javascript/pdfjs-dist--build--pdf.min.mjs.js) since no version is specified in the importmap
Ignoring pdfjs-dist/build/pdf.worker.min.mjs (vendor/javascript/pdfjs-dist--build--pdf.worker.min.mjs.js) since no version is specified in the importmap

This PR makes a change to use the extract_base_package_name() method within find_unversioned_vendored_package() to make it consistent with packages_with_versions() and eliminate the erroneous warning.

I also created a new test to confirm that there is no longer a warning emitted.

@rafaelfranca please let me know if you prefer a different approach.