11948 Use pre-existing local file for import

[paul-m]

• Since it was unclear exactly which layer should contain this solution, I ended up reading a lot of code and thus doing minor refactoring in some places, and adding a lot of docblocks and CS fixes for some files.
  • Notably, DataResource now has getUniqueIdentifierNoPerspective(), to go along with its existing getUniqueIdentifier(), and updating places where the no perspective id was being created on the fly.
  • This all works towards making the whole thing more readable overall.
• Adds a new DKAN common config called always_use_existing_local_perspective. It’s a bool, and makes your DKAN override the file fetcher to use an existing file rather than copy one from the source.
• Adds a warning to the site status page telling you that you have always_use_existing_local_perspective enabled.
• \Drupal\common\FileFetcher\Factory is now \Drupal\common\FileFetcher\FileFetcherFactory. This name change comes along with the ability to check the always_use_existing_local_perspective configuration and use our new file-fetcher remote processor depending on what it says.
• Adds \Drupal\common\FileFetcher\FileFetcherRemoteUseExisting, which is a file fetcher processor to replace Remote if we want to use the existing file.
• Adds tests to prove all this stuff does what it says.
• Added a page of documentation: https://dkan--3996.org.readthedocs.build/en/3996/user-guide/guide_local_files.html

There's no integrity check performed on the file, other than whether it exists in the file system... For instance the file could exist but be zero bytes long. We'll follow up with issues about this process.

[dafeder]

Amazing work here @paul-m. Take a look at a few suggestions and questions I had.

[dafeder]

Approved, looks like we need to re-run a cypress test though