feat(npm): embed npm binary

BREAKING CHANGE: npx will no longer use the system npm -- it embeds its own

index.js

@@ -12,6 +12,7 @@ let rimraf
 const updateNotifier = require('update-notifier')
 const which = BB.promisify(require('which'))
 
+const INSTALLER_PATH = path.resolve(__dirname, 'node_modules/.bin/npm')
 const PATH_SEP = process.platform === 'win32' ? ';' : ':'
 
 updateNotifier({pkg}).notify()
@@ -98,13 +99,12 @@ function getExistingPath (command, opts) {
 
 module.exports._getNpmCache = getNpmCache
 function getNpmCache (opts) {
-  return which('npm').then(npmPath => {
-    const args = ['config', 'get', 'cache']
-    if (opts.userconfig) {
-      args.push('--userconfig', opts.userconfig)
-    }
-    return child.exec(npmPath, ['config', 'get', 'cache'])
-  }).then(cache => cache.trim())
+  const args = ['config', 'get', 'cache']
+  if (opts.userconfig) {
+    args.push('--userconfig', opts.userconfig)
+  }
+  return child.exec(INSTALLER_PATH, ['config', 'get', 'cache'])
+  .then(cache => cache.trim())
 }
 
 module.exports._buildArgs = buildArgs
@@ -121,14 +121,12 @@ function buildArgs (specs, prefix, opts) {
 module.exports._installPackages = installPackages
 function installPackages (specs, prefix, npmOpts) {
   const args = buildArgs(specs, prefix, npmOpts)
-  return which('npm').then(npmPath => {
-    return child.spawn(npmPath, args, {
-      stdio: [0, 'ignore', 2]
-    }).catch(err => {
-      if (err.exitCode) {
-        err.message = `Install for ${specs} failed with code ${err.exitCode}`
-      }
-      throw err
-    })
+  return child.spawn(INSTALLER_PATH, args, {
+    stdio: [0, 'ignore', 2]
+  }).catch(err => {
+    if (err.exitCode) {
+      err.message = `Install for ${specs} failed with code ${err.exitCode}`
+    }
+    throw err
   })
 }