Prevents this crash:
```
$ beet import ~/Music/Music/_/[1405]/00.mp3
Traceback (most recent call last):
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/bin/.beet-wrapped", line 9, in <module>
sys.exit(main())
~~~~^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/ui/__init__.py", line 1859, in main
_raw_main(args)
~~~~~~~~~^^^^^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/ui/__init__.py", line 1838, in _raw_main
subcommand.func(lib, suboptions, subargs)
~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/ui/commands.py", line 1390, in import_func
import_files(lib, byte_paths, query)
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/ui/commands.py", line 1330, in import_files
session.run()
~~~~~~~~~~~^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/importer/session.py", line 234, in run
pl.run_parallel(QUEUE_SIZE)
~~~~~~~~~~~~~~~^^^^^^^^^^^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/util/pipeline.py", line 471, in run_parallel
raise exc_info[1].with_traceback(exc_info[2])
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/util/pipeline.py", line 336, in run
out = self.coro.send(msg)
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/util/pipeline.py", line 219, in coro
func(*(args + (task,)))
~~~~^^^^^^^^^^^^^^^^^^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/importer/stages.py", line 141, in lookup_candidates
plugins.send("import_task_start", session=session, task=task)
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/plugins.py", line 505, in send
result = handler(**arguments)
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beets/plugins.py", line 200, in wrapper
return func(*args, **kwargs)
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beetsplug/fromfilename.py", line 165, in filename_task
apply_matches(d, self._log)
~~~~~~~~~~~~~^^^^^^^^^^^^^^
File "/nix/store/lfv9ns20hz2bg6d44js378vcxjfm9261-beets-2.3.1/lib/python3.13/site-packages/beetsplug/fromfilename.py", line 124, in apply_matches
item.title = str(d[item][title_field])
~~~~~~~^^^^^^^^^^^^^
KeyError: 'title'
```
Centralise plugin loading in `beets.plugins` and refactor the plugin
loading system to be more straightforward and eliminate complex mocking
in tests. Replace the two-stage class collection and instantiation
process with direct instance creation and storage.
Add plugins.PluginImportError and adjust plugin import tests to only
complain about plugin import issues.
Add `py.typed` marker file to support PEP 561 typing
This PR adds a `py.typed` marker file to the package directory to
indicate that the package includes inline type hints and is PEP 561
compliant.
