feat(gatsby-source-wordpress): use unique multi-site node id's (#12683)

## Description

There was a little bug where in a multi-site config of the gatsby-source-wordpress plugin, the id's of the graphQL nodes were not always unique. Since the node id's were mainly being generated by the node.__type, in the case of two post types had the same type and id (even though they were from different data sources), resulted in only 1 graphQL node being created. This PR adds the __siteUrl to the id generation to resolve this issue. 

(My previous PR needed to pull upstream changes, hopefully all circleci tests will pass)
## Related Issues

The changes in this PR will resolve this issue where [multi-site WordPress config looses data](#12608)

Author: sarahannnicholson

packages/gatsby-source-wordpress/src/__tests__/normalize.js

@@ -25,7 +25,11 @@ describe(`Process WordPress data`, () => {
   it(`creates Gatsby IDs for each entity`, () => {
     const createNodeId = jest.fn()
     createNodeId.mockReturnValue(`uuid-from-gatsby`)
-    entities = normalize.createGatsbyIds(createNodeId, entities)
+    entities = normalize.createGatsbyIds(
+      createNodeId,
+      entities,
+      `http://dev-gatbsyjswp.pantheonsite.io`
+    )
     expect(entities).toMatchSnapshot()
   })
   it(`Creates map of types`, () => {

packages/gatsby-source-wordpress/src/gatsby-node.js

@@ -96,7 +96,7 @@ exports.sourceNodes = async (
   entities = normalize.excludeUnknownEntities(entities)
 
   // Creates Gatsby IDs for each entity
-  entities = normalize.createGatsbyIds(createNodeId, entities)
+  entities = normalize.createGatsbyIds(createNodeId, entities, _siteURL)
 
   // Creates links between authors and user entities
   entities = normalize.mapAuthorsToUsers(entities)

packages/gatsby-source-wordpress/src/normalize.js

@@ -179,12 +179,14 @@ exports.excludeUnknownEntities = entities =>
 // Create node ID from known entities
 // excludeUnknownEntities whitelisted types don't contain a wordpress_id
 // we create the node ID based upon type if the wordpress_id doesn't exist
-exports.createGatsbyIds = (createNodeId, entities) =>
+exports.createGatsbyIds = (createNodeId, entities, _siteURL) =>
   entities.map(e => {
     if (e.wordpress_id) {
-      e.id = createNodeId(`${e.__type}-${e.wordpress_id.toString()}`)
+      e.id = createNodeId(
+        `${e.__type}-${e.wordpress_id.toString()}-${_siteURL}`
+      )
     } else {
-      e.id = createNodeId(e.__type)
+      e.id = createNodeId(`${e.__type}-${_siteURL}`)
     }
     return e
   })