Connectors: Refine plugin install check and PHPStan types.

Replace `file_exists()` with `validate_plugin()` in the connectors script module data, and refine PHPStan type definitions in the connectors code to better reflect the actual shape of registered connector data.

Follow-up to [62288].

Props jorgefilipecosta, mukesh27, peterwilsoncc, westonruter, wildworks.
See #65020.

git-svn-id: https://develop.svn.wordpress.org/trunk@62332 602fd350-edb4-49c9-b593-d223f7449a82