Added execution of maven-enforcer-plugin and its bannedDependencies rule to check our set of transitive dependencies against the whitelist of dependencies approved via CQs

Fixed dependency on slf4j-api to use proper version 1.6.2 as requested via CQ #5627

Added TransferEvent.RequestType.GET_EXISTENCE to allow transfer listeners to distinguish GET and HEAD requests, e.g. to provide better log messages

support modules url inheritance

Moved contents of aether-test-util module into internal package, this is no API stuff, at least not yet