Make pluginpath configuration useable

3b39e0f implements tests that show, that the `pluginpath` configuration does not work.
a4b6976 fixes that. It also changes the definition of the `pluginpath` option. This new definition has two advantages:

1.  It works well with the implementation, especially the `__path__` property of a module.

1.  The new setting is more general and does not make additional assumptions. In particular, if by the old definition the `web` plugin was stored under `/path/to/beetsplug/web.py` with plugin path `/path/to/` we just migrate by using the plugin path `/path/to/beetsplug`.

**Note**: I am not very familiar with `.rst` and *Read the docs*, so somebody should check the formatting.