Moved Plexus support components of wagon connector into internal package, nothing but an IoC container should ever access those classes

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