(There are documentation changes and new fields in descriptor.proto that have resulted
in changes to the serialized descriptor, but no breaking changes for C#.)

JSON tests fail, as we're not using OriginalNameAttribute yet.

(Generated code changes in next commit.)

Generated code changes for previous commit (basically InternalBuildGeneratedFileFrom => FromGeneratedCode)

Other changes are due to the well-known types changing without us regenerating.

We now do this in protoc instead of the generation simpler.

Benefits:
- Generation script is simpler
- Detection is simpler as we now only need to care about one filename
- The embedded descriptor knows itself as "google/protobuf/descriptor.proto" avoiding dependency issues

This PR also makes the "invalid dependency" exception clearer in terms of expected and actual dependencies.

We now do this in protoc instead of the generation simpler.

Benefits:
- Generation script is simpler
- Detection is simpler as we now only need to care about one filename
- The embedded descriptor knows itself as "google/protobuf/descriptor.proto" avoiding dependency issues

This PR also makes the "invalid dependency" exception clearer in terms of expected and actual dependencies.

Instead, introduce GeneratedCodeInfo which passes in what we need, and adjust the codegen to take account of this.

We'll see what I've missed when CI fails...

Change the C# namespace in descriptor.proto to Google.Protobuf.Reflection.
This then means changing where the generated code lives, which means updating the project file...

It also involves regenerating the C++ - which has updated the well-known types as well,
for no terribly obvious reason...

Generated code changes from previous commit

Note that now we need a proto3 version of addressbook.proto. This may affect other platforms, and could do with an overhaul to follow proto3 conventions anyway (e.g. repeated field names). Will need to think about that carefully before merging into master. Raised issue #565 for this.

Prod code works, but some tests are broken. Obviously those need fixing, then more tests,
and review benchmarks.

(Primarily this is starting the hash code of messages at a non-zero value...)

Fixes issue #523.