jsonschema: resolve remote references

Support remote references.
Most of the machinery was in place; we just had to define the
right loader, copy files from the test repo, and download
the meta-schema files from json-schema.org.

Change-Id: Ib588200ef0fa483641b9b53f9904f53b39dc4b82
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

jsonschema/testdata/remotes/draft2020-12/format-assertion-false.json

@@ -0,0 +1,13 @@
+{
+    "$id": "http://localhost:1234/draft2020-12/format-assertion-false.json",
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$vocabulary": {
+        "https://json-schema.org/draft/2020-12/vocab/core": true,
+        "https://json-schema.org/draft/2020-12/vocab/format-assertion": false
+    },
+    "$dynamicAnchor": "meta",
+    "allOf": [
+        { "$ref": "https://json-schema.org/draft/2020-12/meta/core" },
+        { "$ref": "https://json-schema.org/draft/2020-12/meta/format-assertion" }
+    ]
+}