MetadataParser: Don't use compilation covers for artists

If they are only appearing in the compilation, don't use the compilation
cover for that artist. Even though we would override it afterward, if
there is no other cover, it's confusing for the compilation to be used
all over the place.
Fix #106
4 jobs for rework_thumbnails_inheritance in 13 minutes and 7 seconds
Status Job ID Name Coverage
  Test
passed #290155
amd64 debian
asan-ubsan

00:13:05

passed #290152
amd64 debian
linux

00:10:27

passed #290153
win32
win32

00:12:28

passed #290154
win64
win64

00:11:49