fetchart: defer file removal config option evaluation

[mgoltzsche]

Description

Defer the evaluation of the source file removal options (import.delete and import.move) to the point where the fetchart plugin is actually called instead of only evaluating those configuration options on plugin initialization. This is to allow other plugins (such as the ytimport plugin) to invoke the import directly (within the same python process; implicitly invoking the fetchart plugin) with temporarily overwritten configuration options.

Addresses https://github.com/beetbox/beets/issues/5167#issuecomment-2106465172

To Do

• [x] ~Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)~
• [x] Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• [x] ~Tests. (Very much encouraged but not strictly required.)~

[mgoltzsche]

I adjusted the test correspondingly now. Please re-review.