Stop calling moduleDoesResolve before installing Cordova npm deps.

This fixes the bug where commands like `meteor add-platform ios` would
fail the first time with an error that cordova-lib could not be found,
even though we attempt to install the necessary packages if they have not
already been installed.

To make a very long story short, calling moduleDoesResolve before
installing dependencies like cordova-lib was causing Node.js to cache the
_absence_ of cordova-lib/package.json permanently in the new
packageJsonCache, which cannot be invalidated or cleared by user code:
https://github.com/nodejs/node/blob/f8f20892e944a6c4b52e298528135161d85fcc7a/lib/internal/modules/cjs/loader.js#L245-L255

Although we could potentially propose a change to Node to allow the
packageJsonCache to be invalidated, a more immediate solution is simply to
avoid calling moduleDoesResolve when there's any chance the module will
not resolve. Because we still want to avoid repeatedly installing Cordova
dependencies every time we run a Cordova command, we instead check whether
the necessary dependencies are installed by examining the file system.

tools/cli/dev-bundle-helpers.js

@@ -1,20 +1,25 @@
-import { pathJoin, getDevBundle } from '../fs/files';
-import { installNpmModule, moduleDoesResolve } from '../isobuild/meteor-npm.js';
+import { pathJoin, getDevBundle, statOrNull } from '../fs/files';
+import { installNpmModule } from '../isobuild/meteor-npm.js';
 
 export function ensureDependencies(deps) {
+  const devBundleLib = pathJoin(getDevBundle(), 'lib');
+  const devBundleNodeModules = pathJoin(devBundleLib, 'node_modules');
+
   // Check if each of the requested dependencies resolves, if not
   // mark them for installation.
   const needToInstall = Object.create(null);
   Object.keys(deps).forEach(dep => {
-    if (!moduleDoesResolve(dep)) {
+    const pkgDir = pathJoin(devBundleNodeModules, dep);
+    const pkgStat = statOrNull(pkgDir);
+    const alreadyInstalled = pkgStat && pkgStat.isDirectory();
+    if (!alreadyInstalled) {
       const versionToInstall = deps[dep];
       needToInstall[dep] = versionToInstall;
     }
   });
 
-  const devBundleLib = pathJoin(getDevBundle(), 'lib');
-
   // Install each of the requested modules.
-  Object.keys(needToInstall)
-    .forEach(dep => installNpmModule(dep, needToInstall[dep], devBundleLib));
+  Object.keys(needToInstall).forEach(dep => {
+    installNpmModule(dep, needToInstall[dep], devBundleLib);
+  });
 }

tools/isobuild/meteor-npm.js

@@ -1053,20 +1053,6 @@ var getShrinkwrappedDependencies = function (dir) {
   return treeToDependencies(getShrinkwrappedDependenciesTree(dir));
 };
 
-const moduleDoesResolve = meteorNpm.moduleDoesResolve = (dep) => {
-  try {
-    require.resolve(dep);
-  } catch (e) {
-    if (e.code !== 'MODULE_NOT_FOUND') {
-      throw e;
-    }
-
-    return false;
-  }
-
-  return true;
-};
-
 const installNpmModule = meteorNpm.installNpmModule = (name, version, dir) => {
   const installArg = utils.isNpmUrl(version)
     ? version