Add special handling for spawning npm

As laid out in istanbuljs/nyc#190, spawn-wrap’ing `npm` does currently
not work when using the `npm` binary that comes bundled with Node.js.

This adds special handler code for the case that `npm` is the
program that should be executed, looking for the `npm` executable
in PATH and invoking the shim with it as an argument.

index.js

@@ -133,6 +133,32 @@ function wrap (argv, env, workingDir) {
       options.envPairs.push((isWindows ? 'Path=' : 'PATH=') + workingDir)
     }
 
+    if (file === 'npm') {
+      var pathDirs = (pathEnv || '').split(colon)
+      var npmPath
+
+      for (var i = 0; i < pathDirs.length; i++) {
+        npmPath = path.resolve(pathDirs[i], 'npm')
+        try {
+          fs.accessSync(npmPath, fs.R_OK | fs.X_OK)
+        } catch (err) {
+          // not found/not executable
+          npmPath = undefined
+          continue
+        }
+
+        // found it
+        break
+      }
+
+      if (npmPath) {
+        options.args[0] = npmPath
+
+        options.file = workingDir + '/node'
+        options.args.unshift(workingDir + '/node')
+      }
+    }
+
     if (isWindows) fixWindowsBins(workingDir, options)
 
     return spawn.call(this, options)