Respect no_convert and never_convert_lossy_files in convert plugin (#6286)

Given that @frigginbrownie's #5556 PR received some thumb ups but they
haven't responded since a while ago, I'm creating this PR to merge their
fix (I could not commit in the PR branch since I have no permissions to
push to their fork).

Supersedes: #5556 

Copying @frigginbrownie description from #5556:

According to the docs, the auto_keep function will "Convert your files
automatically on import to dest but import the non transcoded version."
This is true but not 100% accurate. In cases where no conversion is
required (say, importing lossy files where there's no need to convert),
auto_keep will copy the files to dest.

This behavior results in duplicate files being created on import when
the auto_keep function is set to yes - a lossy file will be imported
into the default directory (say /music) and then copied to the dest
location (say /transcodes).

This is ideal if you wish to have all music formats in your default
directory (lossy and lossless) and all lossy files (original imports and
transcodes) in a secondary directory (say /lossy).

But what if you want a separate directory of all music you've
transcoded? auto_keep won't provide that, as it copies lossy files to
the dest location. In addition, if the dest is set to the same location
as default directory, auto_keep will copy lossy files into the same
directory that beets previously imported files into, resulting in the
directory having two files for each file in an album. If you use paths
(say to have singletons imported into /music/singles), auto_keep will
import the file into the path location, then copy the file to the dest,
creating directories to match the path.

Unlike with the auto option or using "beet convert", auto_keep does not
follow the never_convert_lossy_files or no_convert options and will not
validate whether files need to be converted or copied on import to dest
- it transcodes or it copies, no questions asked.

This change updates the auto_convert_keep function to filter items using
should_transcode. This way, if the user sets never_convert_lossy_files
to no or no_convert: 'format:mp3', lossy files will not be copied to the
dest, while lossless files will be converted to the dest (perfect for a
seperate /transcodes directory). If the user sets
never_convert_lossy_files to yes, lossy files will to be copied to the
dest and lossless files will be converted to the dest (perfect for a
/lossy directory). In turn, this change makes behavior consistent with
"beet convert" and the auto option.

Author: snejus

beetsplug/convert.py

@@ -279,6 +279,10 @@ def auto_convert_keep(self, config, task):
             ) = self._get_opts_and_config(empty_opts)
 
             items = task.imported_items()
+
+            # Filter items based on should_transcode function
+            items = [item for item in items if should_transcode(item, fmt)]
+
             self._parallel_convert(
                 dest,
                 False,

docs/changelog.rst

@@ -92,6 +92,9 @@ Bug fixes:
   preventing incorrect splits when both are present.
 - :doc:`reference/cli`: Fix 'from_scratch' option for singleton imports: delete
   all (old) metadata when new metadata is applied. :bug:`3706`
+- :doc:`/plugins/convert`: ``auto_keep`` now respects ``no_convert`` and
+  ``never_convert_lossy_files`` when deciding whether to copy/transcode items,
+  avoiding extra lossy duplicates.
 
 For plugin developers: