Enforce that the file name doesn’t collide with any of its messages

The Google protobuf generator for Java creates a class for each .proto, and a nested class for each message in that .proto. So if we define say, Dinosaur.proto like this:

...
message Dinosaur {
}

Then the Google generator generates Dinosaur.java that looks like this:

...
class Dinosaur {
  class Dinosaur {
  }
}

And then javac refuses to compile this, because a class cannot have the same name as its enclosing class.

Reasonable people have been making this mistake in projects that only use Wire. But then when they attempt to use protobuf their builds fail because of the Google protobuf limitation.

We should detect this problem in Wire and report an error.

Reproduce the example from Dinosaur.proto and inspect the generated Dinosaur.java, focusing on the Java generation path and how Wire reports invalid schema definitions. The work is done when a message sharing the .proto file name is detected before generation and reported as an error, preventing the javac failure.