Use custom serialization for fileobject subclasses.

[sdedic]

When LSP client issues project metadata command to inspect project structure / produced artifacts, the ArtifactSpec may containa reference to a FileObject, if the artifact was built already. The original idea was to serialize FileObjects as paths, but I made a wrong test -- the check should accept all FileObject subclasses.
In this case a MultiFileObject was serialized from Maven ArtifactSpec resulting in a serialization error:

@b18a155
	at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315)
	at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:649)
	at java.base/java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:482)
	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1418)
	at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:45)
	at org.openide.util.lookup.Lookups.executeWith(Lookups.java:278)
	at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2033)
Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make field private final java.util.concurrent.locks.ReentrantLock$Sync java.util.concurrent.locks.ReentrantLock.sync accessible: module java.base does not "opens java.util.concurrent.locks" to unnamed module @b18a155
	at java.base/java.lang.reflect.AccessibleObject.throwInaccessibleObjectException(AccessibleObject.java:387)
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:363)
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:311)
	at java.base/java.lang.reflect.Field.checkCanSetAccessible(Field.java:180)
	at java.base/java.lang.reflect.Field.setAccessible(Field.java:174)
	at com.google.gson.internal.reflect.UnsafeReflectionAccessor.makeAccessible(UnsafeReflectionAccessor.java:44)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:159)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
	at com.google.gson.Gson.getAdapter(Gson.java:489)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.createBoundField(ReflectiveTypeAdapterFactory.java:117)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:166)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:102)
	at com.google.gson.Gson.getAdapter(Gson.java:489)

since GSON serializer traversed deep into the FileObject.

[jhorvath]

Looks good