Check source modified dates in convert plugin

If you convert a track and then modify the metadata of the original track the beets convert plugin doesn't realise that it needs to reconvert the file because it only checks if the file already exists.

Mature conversion tools also check the modified date of the source file so that even if the transcoded destination file already exists if the modified date of the source file is newer it gets reconverted.

The issue names the beets convert plugin but no files or tests. Trace that plugin's decision about an existing destination, then inspect how source and destination modification times are available. Done means a newer source metadata modification causes reconversion even when the destination already exists, with coverage for that case.

Written by the indexing model from the issue text.