Deleted superfluous StubArtifact and StubMetadata now that DefaultArtifact and DefaultMetadata are part of API

Configured snapshot/release distribution via oss.sonatype.org (cf. https://issues.sonatype.org/browse/OSSRH-2684)

Tweaked configuration for maven-bundle-plugin to workaround bogus warning about superfluous export package instructions (https://issues.apache.org/jira/browse/FELIX-3300)

Changed approach to use packaging=jar and only have the maven-bundle-plugin produce the MANIFEST.MF. For unknown reasons, having maven-bundle-plugin build the JARs when packaging=bundle will exclude target/classes/META-INF/* from the JAR, thereby killing Plexus/Sisu support.

Moved DefaultArtifact, DefaultArtifactType, DefaultMetadata and DefaultDependencyNode into API to improve usability

Tweaked configuration for maven-bundle-plugin to produce Bundle-Name header confirming with incubation requirements

Added missing optional markers for aether-connector-asynchttpclient bundle

Moved DefaultRepositorySystemSession, DefaultRepositoryCache and DefaultSessionData over into API module to improve usuability

- Made get*Properties() provide read-only views
- Made copy constructor clone incoming maps to avoid unintended data coupling with input session

Refactored DefaultServiceLocator to use composition rather than inheritance to support error handling hook