There are two definitions for distribution.proto: one in googleapis/googleapis,
and another in googleapis/api-common-protos. The canonical source is supposed
to be in the latter but is actually in the former. I've manually deleted
the distribution.proto in api-commons-proto on my local system and re-run
regen.sh to force the update in this PR.

This makes it easier to reason about what this script is doing.

Mostly for texttospeech, per internal bug 122452555.

While we're in the $GOPATH transitionary period, a dependency could
accidentally find its way into our require statements without being codified in
go.mod/go.sum. This CL makes our CI check for such a case, and exit 1 if it
detects it.

Mostly for the new firestore/v1 protos

This reverts commit 73d5040b.

This does a regeneration using the version of protobuf specified in the
go.mod: v1.2.0.

This reverts commit 3360461a.

This commit was generated by go mod init && go mod tidy

This regeneration pulls in the latest generator
change that deprecates x/net/context in favour
of context. Since we no longer support Go
versions below 1.9, context is preferred.

regen

* regenerate

* using latest protoc-gen-go

should be fixed in https://github.com/grpc/grpc-go/pull/2242#pullrequestreview-140912658 (#86)