This change makes use of new imports attribute for Bazel's Python rules, which
enable adding directories to the PYTHONPATH. This allows us to remove
the hack for building protobuf's Python support with Bazel and now
allows projects to include protobuf using a Bazel external repository
rather than requiring it to be imported directly into the source tree as
//google/protobuf.

This change also updates the protobuf BUILD file to use a named
repository, @python//, for including Python headers rather than
//util/python. This allows projects to specify their own package for
Python headers when including protobuf with an external repository.

Fixes #1230

--allow_oversize_protos=true is passed to bazel build.
When this macro is set, SetTotalBytesLimit is called to remove
the 64MB limit on binary protos when during ParseFromString.

Change-Id: I6fa9f1b65d6c06b891aabf3f3d868364d74d727d

Fixes #1029

The sources themselves appear to already be Python 3 clean.

This changes csharp_names.h, which will require a corresponding change in GRPC.

This adds implicit dependencies to the coressponding runtime libraries.

We need to use the list to indicate field presense.
The field must only contain 0 or 1 string element.

This is needed to support python bazel build.

library.

When trying to compile the protobuf code as a DLL, and then compile other DLLs
with generated .pb.cc/h files that reference
InternalMetadataWithArena::InternalMetadataWithArena(Arena*), MSVC gives an
"unresolved external symbol" error.  This seems to be due to the function being
simultaneously exported and inline.  Moving it out-of-line fixes things.

There are other functions exported and inline as well but de-inlining them
doesn't seem to be necessary to get the build working, and I'd rather de-inline
as few functions as possible.

Change-Id: I17e16fdae6e4d2fb74d178fa5564a609ed58af1d

Also add WKT headers to the umbrella file, and simplify Podspec with it.
Plus some layout improvements to the BUILD file.

Change-Id: I63ed5fb58a45e098f2fd1fa457a219148de030bd

Induce those from the input proto file names instead to reduce the
lenght of the BUILD file.

Change-Id: I1dda57dedac547f4bd0cc3a7d08fb611d72acd13

Change-Id: I0b5a2c017f5412aaca3a47815671753cdef63165

Change-Id: I62640af99e3c2650a29dc5468ce3deb7b6c7efdf

Change-Id: I5a299d969ff96d7d2f80aadc7e8987d461d24b8f