The Picard installer (at least on Windows) installs all contributed plugins into the plugins subdirectory of the install directory - however it does NOT respect the directory structure in the Picard plugins/contrib source directory so e.g.

/lastfm/_init_.py
/lastfm/ui_options_lastfm.py
/lastfmplus/_init_.py
/lastfmplus/ui_options_lastfm.py
/replaygain/_init_.py
/replaygain/ui_options_replaygain.py
/viewvariables/_init_.py
/viewvariables/ui_variables_dialog.py

are all copied to:

C:\Program Files (x86)\MusicBrainz Picard\plugins

As you might guess, this creates a problem because:

1. e.g. _init_.py is used multiple times and only the last file copied remains; and

2. Picard treats e.g. ui_options_lastfm.py as a plugin (rather than a file used by a plugin) and then gives errors.