Update C++ code generation to work with Bazel 0.29 .

[lberki]

The above Bazel version changes proto compilation slightly: some proto
files are put into a _virtual_imports directory and thus
_get_include_directory needs to be updated accordingly.

Ideally, it would use instead the ProtoInfo provider to tease out the
proto import directories, but that's a bit more intrusive change.

[lberki]

This is to fix bazelbuild/bazel#9032 . It should be benign, since previous Bazel versions did not use the _virtual_imports directory.

[gnossen]

Glad to see bazelbuild/bazel#7157 is getting taken care of. Thanks for the PR!

[gnossen]

Existing failures:

• bm_chttp2_transport --benchmark_filter=BM_TransportStreamSend/134217728$ GRPC_POLL_STRATEGY=poll #18416
• TEST TIMEOUT: grpc/core/pull_request/macos/grpc_basictests_node #19669
• dart server failed in interop tests #18616

Still need a review from a codeowner. @jtattermusch @nicolasnoble ?

[lberki]

@gnossen , do I understand correctly that the above breakages are also at HEAD and thus okay?

[lberki]

It's my job to take care of these things! Unfortunately, this time, the collateral damage was greater than zero but it also made the way proto_library works significantly nicer, so there should be less breakages in the future.

[gnossen]

@lberki Yes. The failures I listed above are known flakes or deterministic failures on master that should be unrelated to the changes in this PR and should not block merging.

[lberki]

(friendly ping)

Note that this is breaking Envoy and google-cloud-cpp, so it would be real nice if this got fixed.

[gnossen]

CC @nicolasnoble @jtattermusch @a11r

[lberki]

(friendly ping)

[lberki]

Thanks! What's the process for merging this? (I don't have a bit fat green "Merge" button...)

[jtattermusch]

Known failures:
#19877
#18616

[lberki]

Thanks! Now on for propagating this downstream (but that's my problem!)