NEXUS-6213: Validation for Maven Site XML

Handling deployed site descriptors too in "maven specific" validation. Also, added a log line when "lax" XML validation kicks in (as log is already confusing, XML is reported as invalid, but processing continues).

Issue
https://issues.sonatype.org/browse/NEXUS-6213

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF6