encoding/openapi: support Definitions

Fixes #131

Change-Id: Ic7581a22dddb0c6b01707c78ea9b051abfa0e0e2
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3584
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/testdata/oneof.cue b/encoding/openapi/testdata/oneof.cue
index d318c50..768b442 100644
--- a/encoding/openapi/testdata/oneof.cue
+++ b/encoding/openapi/testdata/oneof.cue
@@ -1,10 +1,10 @@
-MyString: {
+MyString :: {
 	exact: string
 } | {
 	regex: string
 }
 
-MyInt: int
+MyInt :: int
 
 Foo: {
 	include: MyString