Remove legacy Xpp3Dom methods out of ArtifactUtils
Remove namespace from Artifact

Namespace is now included in ArtifactMetadata.
Remove stereotype field from Artifact
Remove remains of stereotype from DefaultArtifact
[test] Fix compiler warnings in locator tests
[refactor] Extract artifact type registry to a new class

This also removes dependency on Aether API.
[refactor] Move Maven model-related code to XMvn MOJO
[refactor] Move repository code to XMvn installer

XMvn core still retains a copy of repository code because it is still
used by deprecated depmap resolver.  In (hopefully near) future this
code will be removed.
[refactor] Move depracated depmap code to separate namespace

This is to make sure it's not confused with currently developed code.
Also copy some other classes so that deprecated code can be left
without change while its dependencies are being refactored.
[test] Commit empty directory needed for classloader tests
[test] Implement PackageRegistryTest
[test] Implement InstallationPlanTest
[refactor] Remove unused Ivy model reader
[test] Fix paths in InstallationPlanTest
[aux] Add site-stage script