Keep directories which contain only *.proto files and subdirectories
Keep directories which contain only *.proto files and subdirectories

These can be referenced by *.proto files kept via b1f824fe5960, which only kept
*.proto located in directories on which something also has a go dependency.

The approach here keeps only *.proto files which are alone in a directory
(apart from other subdirectories), this retains protobuf include dirs without
pulling in lots of other stuff, but assumes that e.g. a foo.proto in a
directory with foo.go is only wanted if foo.go is.

The example I tripped over was vendoring github.com/gogo/protobuf where
github.com/gogo/protobuf/gogoproto/gogo.proto imports
google/protobuf/descriptor.proto but
github.com/gogo/protobuf/protobuf/google/protobuf/descriptor.proto is cleaned
away (as would github.com/google/protobuf's version)

Signed-off-by: Ian Campbell <9be92b3dbbd6611e3e4c9209fb3d04b4919e7cca@docker.com>

LK4D4/vndr