fetchart: defer file removal config option eval

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](https://github.com/mgoltzsche/beets-ytimport/blob/v1.8.1/beetsplug/ytimport/__init__.py#L194) plugin) to invoke the import directly (within the same python process; implicitly invoking the fetchart plugin) with temporarily overwritten configuration options.

Relates to #5167 (comment)

beetsplug/fetchart.py

@@ -1220,10 +1220,6 @@ def __init__(self):
         self.cautious = self.config["cautious"].get(bool)
         self.store_source = self.config["store_source"].get(bool)
 
-        self.src_removed = config["import"]["delete"].get(bool) or config[
-            "import"
-        ]["move"].get(bool)
-
         self.cover_format = self.config["cover_format"].get(
             confuse.Optional(str)
         )
@@ -1307,10 +1303,11 @@ def assign_art(self, session, task):
         """Place the discovered art in the filesystem."""
         if task in self.art_candidates:
             candidate = self.art_candidates.pop(task)
+            removal_enabled = _is_source_file_removal_enabled()
 
-            self._set_art(task.album, candidate, not self.src_removed)
+            self._set_art(task.album, candidate, not removal_enabled)
 
-            if self.src_removed:
+            if removal_enabled:
                 task.prune(candidate.path)
 
     # Manual album art fetching.
@@ -1410,3 +1407,8 @@ def batch_fetch_art(self, lib, albums, force, quiet):
                 else:
                     message = ui.colorize("text_error", "no art found")
                 self._log.info("{0}: {1}", album, message)
+
+def _is_source_file_removal_enabled():
+    delete_enabled = config["import"]["delete"].get(bool)
+    move_enabled = config["import"]["move"].get(bool)
+    return delete_enabled or move_enabled

docs/changelog.rst

@@ -163,6 +163,8 @@ New features:
 * Add support for `barcode` field.
   :bug:`3172`
 * :doc:`/plugins/smartplaylist`: Add new config option `smartplaylist.fields`.
+* :doc:`/plugins/fetchart`: Defer source removal config option evaluation to
+  the point where they are used really, supporting temporary config changes.
 
 Bug fixes: