Fix test for unexpected type url when parsing Any. Currently, the test fails since TestAllTypes doesn't have field '@type', which is the same test as testUnknownFields.

Fix duplicate fields test. The repeated version is passing because null values in a repeated field are rejected and not testing what it wanted to. Also adds a oneof version that verifies the case of oneof fields of different names (currently only same name check seems to be tested).

Also fix spelling of a test.

The default behaviour of throwing an exception remains

This includes all internal changes from around May 20 to now.

- Using custom URL for types in Any will no longer throw an
InvalidProtocolBufferException in JsonFormat
- Fixes #1128

Java files are moved to un-do the hack in the prevous commit, which
moved the java files to the original position for integration.

Moving the files to their original location, so that opensource changes
can be picked during the internal merge. Those files will be moved into
the correct location after merging with internal code.

Note: do NOT merge this into master without the other internal
down-integration commit.

protobuf/java will become a parent pom that will contain two modules:

core - contains all of the code for the protobuf-java artifact
util - contains all of the code for the protobuf-java-util artifact

Also cleaned up various Maven warnings.

Change-Id: I3daf32cb93dfe0fca38f8b48615ccfc307f748e9