Vendor click as dependency for Metaflow

[savingoyal]

Additionally -

1. Removes coverage and click from Metaflow's pinned dependency requirements for @conda and floats other dependencies.
2. This PR will result in dependency re-resolution for all workflows that rely on @conda. We can handle it as a special case but the effort/complexity may not be worth it.
   As a side effect this PR now ensures Metaflow @conda works with Py3.9

[romain-intel]

This LGTM with the following comments:

• LICENSES need to be included (it's a requirement of the license) for both click and importlib_metadata
• As discussed, we should roll back importlib_metadata so it supports Py 3.6
• I wonder if the coverage test in formatter.py should exclude _vendor (is it present)?

[savingoyal]

Added LICENSES for click, importlib_metadata and their dependencies. vendor.py automatically takes care of that - we would need to add more edge cases for LICENSE detection in case we add more dependencies into the mix. Rolled back both click and importlib_metadata.