cleanup of websocket to fix JPMS warnings

[lachlan-roberts]

remove the "export to" lines from websocket internal packages
move some classes out of the internal packages so they are now visible

[sbordet]

@lachlan-roberts also, jetty-websocket-core-client has this warning:

[INFO] --- maven-bundle-plugin:5.1.8:manifest (default) @ jetty-websocket-core-client ---
[INFO] No MANIFEST.MF file found, generating manifest.
[WARNING] Manifest org.eclipse.jetty.websocket:jetty-websocket-core-client:jar:12.0.0-SNAPSHOT : Unused Export-Package instructions: [org.eclipse.jetty.websocket.core.client.internal.*] 
[INFO] Writing manifest: /home/simon/opensource/jetty/12.0/jetty-core/jetty-websocket/jetty-websocket-core-client/target/classes/META-INF/MANIFEST.MF

Something related to #9327?

[gregw]

this PR should also remove the work around from the OSGI manifest.

[janbartel]

OSGi workaround lines to remove:

./jetty-websocket-core-common/pom.xml: ,org.eclipse.jetty.websocket.core.common.internal.
./jetty-websocket-core-client/pom.xml: ,org.eclipse.jetty.websocket.core.client.internal.