fix(gatsby-plugin-mdx): Use getNodesByType for plugin transformation (#22555)

Since getSourcePluginsAsRemarkPlugins is called on
every MDX file, the getNodes call became expensive
at scale with thousands of MDX files.

This was encountered when attempting to find
underlying issues for #22521. @pvdz had a hunch that
node traversal may be a culprit. This doesn't fully
address all underlying performance issues but
is a quick win.

8k MDX pages: 440s => 350s
16k MDX pages: 1100s => 750s

Author: johno

packages/gatsby-plugin-mdx/loaders/mdx-loader.test.js

@@ -74,7 +74,10 @@ describe(`mdx-loader`, () => {
           }
         },
         query: {
-          getNodes() {
+          getNodes(_type) {
+            return fixtures.map(([, node]) => node)
+          },
+          getNodesByType(_type) {
             return fixtures.map(([, node]) => node)
           },
           pluginOptions: {},

packages/gatsby-plugin-mdx/utils/get-source-plugins-as-remark-plugins.js

@@ -13,7 +13,7 @@ module.exports = async function getSourcePluginsAsRemarkPlugins({
   gatsbyRemarkPlugins,
   mdxNode,
   getNode,
-  getNodes,
+  getNodesByType,
   reporter,
   cache,
   pathPrefix,
@@ -39,7 +39,7 @@ module.exports = async function getSourcePluginsAsRemarkPlugins({
       }
   }
 
-  fileNodes = getNodes().filter(n => n.internal.type === `File`)
+  fileNodes = getNodesByType(`File`)
 
   // return list of remarkPlugins
   const userPlugins = gatsbyRemarkPlugins