Search for types loaded by StartupClassLoader in the OSGi framework
bundle. Most of the boot classpath should be contained in
org.eclipse.osgi or its fragments. If this approximation doesn't find
anything, the slower fallback to AbstractSourceLookupDirector will take
care of it.

Change-Id: I1098f3a47b3e01bcdb853c58a2b050600a42ad65
Signed-off-by: Julian Honnen <julian.honnen@vector.com>