Adds a rootDir config option used to resolve plugins

packages/gatsby/src/bootstrap/get-config-file.js

@@ -49,5 +49,7 @@ module.exports = async function getConfigFile(
     }
   }
 
-  return configModule
+  return Object.assign(configModule, {
+    rootDir,
+  })
 }

packages/gatsby/src/bootstrap/load-plugins/load.js

@@ -8,6 +8,7 @@ const { warnOnIncompatiblePeerDependency } = require(`./validate`)
 const { store } = require(`../../redux`)
 const existsSync = require(`fs-exists-cached`).sync
 const createNodeId = require(`../../utils/create-node-id`)
+const createRequireFromPath = require(`../../utils/create-require-from-path`)
 
 function createFileContentHash(root, globPattern) {
   const hash = crypto.createHash(`md5`)
@@ -46,9 +47,11 @@ const createPluginId = (name, pluginObject = null) =>
  * This can be a name of a local plugin, the name of a plugin located in
  * node_modules, or a Gatsby internal plugin. In the last case the pluginName
  * will be an absolute path.
+ * @param {string} rootDir
+ * This is the project location, from which are found the plugins
  * @return {PluginInfo}
  */
-function resolvePlugin(pluginName) {
+function resolvePlugin(pluginName, rootDir) {
   // Only find plugins when we're not given an absolute path
   if (!existsSync(pluginName)) {
     // Find the plugin in the local plugins folder
@@ -81,7 +84,8 @@ function resolvePlugin(pluginName) {
    * which should be located in node_modules.
    */
   try {
-    const resolvedPath = slash(path.dirname(require.resolve(pluginName)))
+    const projectRequire = createRequireFromPath(`${rootDir}/:internal:`)
+    const resolvedPath = slash(path.dirname(projectRequire.resolve(pluginName)))
 
     const packageJSON = JSON.parse(
       fs.readFileSync(`${resolvedPath}/package.json`, `utf-8`)
@@ -110,7 +114,7 @@ module.exports = (config = {}) => {
   // Also test adding to redux store.
   const processPlugin = plugin => {
     if (_.isString(plugin)) {
-      const info = resolvePlugin(plugin)
+      const info = resolvePlugin(plugin, config.rootDir)
 
       return {
         ...info,

packages/gatsby/src/joi-schemas/joi.js

@@ -2,6 +2,7 @@ const Joi = require(`joi`)
 
 export const gatsbyConfigSchema = Joi.object().keys({
   __experimentalThemes: Joi.array(),
+  rootDir: Joi.string(),
   polyfill: Joi.boolean(),
   siteMetadata: Joi.object(),
   pathPrefix: Joi.string(),

packages/gatsby/src/utils/create-require-from-path.js

@@ -0,0 +1,15 @@
+const Module = require(`module`)
+const path = require(`path`)
+
+// Polyfill Node's `Module.createRequireFromPath` if not present (added in Node v10.12.0)
+module.exports =
+  Module.createRequireFromPath ||
+  function(filename) {
+    const mod = new Module(filename, null)
+
+    mod.filename = filename
+    mod.paths = Module._nodeModulePaths(path.dirname(filename))
+    mod._compile(`module.exports = require;`, filename)
+
+    return mod.exports
+  }