Untabify sources
Fallback to "jar" extension when resolving POMs

For non-POM modules (packaging other than "pom") POM artifacts don't
have their own depmaps, but use extension "jar" instead.

Thus when translating POM artifacts using depmaps first try normal
extension (i.e. "pom") and fallback to "jar".
Update DependencyMap interface

Reimplement depmap translation as transitive closure instead of
reflective transitive closure of mappings.
Add required empty repositories to default configuration

Without this, if there are no required repos in user-provided config
then XMvn would fail with ugly "Guice provision error".