Appending module identifier extensions should not match directories.

This logic needed to change not only in meteor/tools/isobuild/resolver.js
but also in the runtime module system:
cfcc422725

Fixes #8539.

packages/modules-runtime/.npm/package/npm-shrinkwrap.json

@@ -1,9 +1,9 @@
 {
   "dependencies": {
     "install": {
-      "version": "0.8.7",
-      "resolved": "https://registry.npmjs.org/install/-/install-0.8.7.tgz",
-      "from": "install@0.8.7"
+      "version": "0.8.8",
+      "resolved": "https://registry.npmjs.org/install/-/install-0.8.8.tgz",
+      "from": "install@0.8.8"
     }
   }
 }

packages/modules-runtime/package.js

@@ -1,13 +1,13 @@
 Package.describe({
   name: "modules-runtime",
-  version: "0.7.9",
+  version: "0.7.10",
   summary: "CommonJS module system",
   git: "https://github.com/benjamn/install",
   documentation: "README.md"
 });
 
 Npm.depends({
-  install: "0.8.7"
+  install: "0.8.8"
 });
 
 Package.onUse(function(api) {

tools/isobuild/resolver.js

@@ -176,7 +176,7 @@ export default class Resolver {
       this.extensions.some(ext => {
         const pathWithExt = path + ext;
         const stat = this.statOrNull(pathWithExt);
-        if (stat) {
+        if (stat && ! stat.isDirectory()) {
           return result = { path: pathWithExt, stat };
         }
       });