Ignore MacOS specific "extended attributes" file during unpack

MacOS tar adds a special file with a name that starts with
"./._" to the root of a tar file. This caused problems when
unpacking modules since a module is supposed to contain one singe
directory in the root of the tar archive.

This commit ensures that the MacOS specific file is ignored.
Handle simplified form for operatingsystem_support in metadata.json

Some modules uses a metadata.json with an operatingsystem_support
array with just strings instead of the stipulated array of objects.
This commit ensures that both formats are handled.