The fetchart plugin would silently drop unknown sources defined in
config, leading to hard to debug problems.
The plugin now errors when an unknown source is configured, or when no
sources are configured.
In addition, a single string is now a valid value for `sources` to
either enable all sources with an `*` or a single source.
Fixes: #6336
Add a `requires_import` pytest marker and `is_importable` fixture to
conditionally skip or adjust assertions based on whether optional
dependencies are available. Apply this to `langdetect`-dependent
language detection tests in lyrics and migration test suites.
