protobuf.bzl: skip '.' when handling includes in _proto_gen_impl()

When generating `-I` arguments from `ctx.attr.includes` in
`_proto_gen_impl()`, we might find '.' in the list. That's effectively
source_dir, which we've already handled earlier, and has more
specialized expansion code. Skip it here.

#test-continuous

PiperOrigin-RevId: 689557912

protobuf.bzl

@@ -99,6 +99,10 @@ def _proto_gen_impl(ctx):
 
     if ctx.attr.includes:
         for include in ctx.attr.includes:
+            if include == ".":
+                # This is effectively source_dir, which has already been handled,
+                # and may be generated incorrectly here.
+                continue
             import_flags += ["-I" + _GetPath(ctx, include)]
 
     import_flags = depset(direct = import_flags)