Selectively add source or gen dir to includes. (#9438)

When both directories are added this results in protoc emitting a
"warning: directory does not exist.". This makes sense because when
there are no inputs from the other directory, it is also not present
n the sandbox where protoc is executed.

protobuf.bzl

@@ -80,7 +80,14 @@ def _proto_gen_impl(ctx):
     source_dir = _SourceDir(ctx)
     gen_dir = _GenDir(ctx).rstrip("/")
     if source_dir:
-        import_flags = depset(direct=["-I" + source_dir, "-I" + gen_dir])
+        has_sources = any([src.is_source for src in srcs])
+        has_generated = any([not src.is_source for src in srcs])
+        import_flags = []
+        if has_sources:
+            import_flags += ["-I" + source_dir]
+        if has_generated:
+            import_flags += ["-I" + gen_dir]
+        import_flags = depset(direct=import_flags)
     else:
         import_flags = depset(direct=["-I."])