jsonschema: resolve URIs for $id

Assign a base URI to every schema.

See the comment on resolveURIs for details.

Change-Id: I2332d825c486766832503cabdd0bf9d819171d53
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669997
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

jsonschema/validate_test.go

@@ -51,7 +51,7 @@ func TestValidate(t *testing.T) {
 			}
 			for _, g := range groups {
 				t.Run(g.Description, func(t *testing.T) {
-					rs, err := g.Schema.Resolve()
+					rs, err := g.Schema.Resolve("")
 					if err != nil {
 						t.Fatal(err)
 					}