.bazelignore the modules/ dir. (#2270)

With the addition of overlays (#1566) it now can make sense to have
BUILD and .bzl files inside the module version directories. However they
don't necessarily make sense on their own. They are only supposed to
work in the context of the source archive they overlay.

To prevent issues like not being able to load() from a //:def.bzl that
does not exist in the registry repo, let's just .bazelignore the
modules/ dir.

.bazelignore

@@ -0,0 +1 @@
+modules/