NEXUS-5418: Fix for MD helper

As the checksum files are not "real" files anymore
(since 2.5), they are stored as attributes in "main" items
themselves. Still, collectionItem#list will return them,
hence walkers like this will try to delete them, resulting
in INFex
Merge remote-tracking branch 'origin/master' into nexus-5418-md-helper-fix
NEXUS-5418: More general fix

Instead fixing all the code that would delete
maven artifact checksums, handle this
change at more general level: at
maven repository level.

With this change, existing code works
"as before", as maven repositories will
stop throwing INFEx at checksum deletion
attempt, while the coll#list still
contains those.
Merge pull request #53 from sonatype/nexus-5418-md-helper-fix

NEXUS-5418: Fix for MD helper