RPM Support

According to information from modularity folks RPMTAG_DISTTAG is no longer in use for modularity purposes.
RPMTAG_MODULARITYLABEL should be used instead.

This affects applicability calculations which rely on the correct identification of modular RPMs.
The problem should not be huge or broad, most modular RPMs are identified as such at sync time using metadata.

This tag is supported in old versions of rpm, just the name is not exposed and numeric key should be used:
The constant rpm. RPMTAG_MODULARITYLABEL is not available on Centos7 and < F30, instead the id if the header 5096 can be used directly: headers[5096].