finalization of third party custom elements support

[tlouisse]

This PR is a continuation of https://github.com/open-wc/custom-elements-manifest/pull/142 (it was almost finished)

As discussed, the commit history of https://github.com/open-wc/custom-elements-manifest/pull/142 was squashed, so it was easier to rebase with master. Because of the latter, this pr goes directly into master.

On top of your squashed commit, it has:

• fix(find-dependencies): basepath support for uptraversal allows to provide cwd (other than process.cwd())
• chore(analyzer): finalize third party manifests finalizes the open pull request
• chore: update tests for 3rd party cems and adjusted cwd](https://github.com/open-wc/custom-elements-manifest/pull/173/commits/816b5c57bf4d46051ad804d17aec8a7d6d924161)

Let me know if you want to organize the commit history in a different way.

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[thepassle]

Could you update the version of find-dependencies in the package.json?

[tlouisse]

@thepassle added a (deliberately failing) test for symlinked monorepo packages

[thepassle]

Thanks for pushing for this and getting it across the finish line!