The conformance tests now use types which are part of src/google/protobuf, so we need to include src in the proto path.
The notes around "fix-ups" have been out of date for some time now.

This introduces a new C# option, base_namespace.
If the option is not specified, the behaviour is as before: no directories are generated.
If the option *is* specified, all C# namespaces must be relative to the base namespace, and the directories are generated relative to that namespace.

Example:
- Any.proto declares csharp_namespace = "Google.Protobuf.WellKnownTypes"
- We build with --csharp_out=Google.Protobuf --csharp_opt=base_namespace=Google.Protobuf
- The Any.cs file is generated in Google.Protobuf/WellKnownTypes (where it currently lives)

We need a change to descriptor.proto before this will all work (it wasn't in the right C# namespace) but that needs the other descriptors to be regenerated too. See next commit...

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.

Completely untested so far - easier to get started in VS and then transfer to Linux for tweaking...

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

This involves:
- Specifying a namespace in each proto (including ones we'd previously missed)
- Updating the generation script
- Changing codegen to implement IReflectedMessage.Fields explicitly (a good thing anyway)
- Changing reflection tests to take account of the explicit interface implementation

Non-generated code in this commit; generated code to follow

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...

- Change the default message hash code to 1 to be consistent with other code
- Change the empty list/map hash code to 0 as "empty map" is equivalent to "no map"
- Removed map fields from unittest_proto3.proto
- Created map_unittest_proto3.proto which is like map_unittest.proto but proto3-only
- Fixed factory methods in FieldCodec highlighted by using all field types :)
- Added tests for map serialization:
  - Extra fields within entries
  - Entries with value then key
  - Non-contiguous entries for the same map
  - Multiple entries for the same key

Changes to generated code coming in next commit

We still have some protos which aren't generated how we want them to be:

- Until we have an option to specify the "umbrella" class, DescriptorProtoFile
  will be broken. (The change of name here affects the reflection descriptor,
  which accounts for most of the change. That's easier than trying to work out
  exactly which occurrences of Descriptor need changing though.)
- That change affects UnittestCustomOptions
- Issue #307 breaks Unittest.cs

After this commit, we don't have the record of the fixups in the files themselves
any more, but one centralized record in the shell script.

To my surprise, executing generate_protos.sh used the version of Bash installed with Git for Windows by default.
After a few modifications to detect the most appropriate protoc to use, this worked pretty simply.
This change also:
- adds generation of the address book tutorial proto,
- fixes the addressbook.proto to specify proto2 explicitly (to avoid a warning from protoc; I don't think we want warnings...)
- fixes the addressbook.proto C# namespace (which I thought I'd done before, but apparently hadn't)
- includes the regenerated UnittestCustomOptions.cs apart from the DescriptorProtoFIle => Descriptor change