Move extensions cache from $extensionsDir to $uploadDir

CiviCRM has been putting cache files in the extensions direction (i.e. cache/timestamp.txt and cache/extensions.json.*).

In some deployments, all extensions are committed to an aggregated git repo, but these files should not be.

In line with other .gitignore files that CiviCRM provides, a .gitignore file should be written to the extensions directory to exclude the cache folder in its entirety.

Move these files to the uploadDir, which is more commonly used for ephemeral data.