feat: wip support for a --from-lockfile flag on npm i

This change needs additional testing to make sure that it is actually respecting
lockfile. It looks for a `--from-lockfile` flag, and if it is detected, it
follows the same codepath as `npm ci`, except without cleaning out the
`node_modules` directory first.

Performance is going to vary wildly depending on what dependencies look like,
but locally in my testing so far I see a roughly 4x speedup over `npm ci`.

Of course, all of this is contingent on more testing to make sure that this
isn't giving incorrect behavior in some way. `npm ci` also relies on explicitly
passing in an option not to modify the lockfile. But I'm unsure of whether
that's necessary here.

And I'm sure that some unit tests are required too.

lib/install.js

@@ -133,7 +133,24 @@ class Install extends BaseCommand {
       path: where,
       add: args,
     }
+
     const arb = new Arborist(opts)
+
+    if (this.npm.config.get('from-lockfile')) {
+      if (args.length) {
+        throw new Error('`--from-lockfile` can not be used to install a new package.');
+      }
+
+      await arb.loadVirtual().catch(er => {
+        log.verbose('loadVirtual', er.stack)
+        const msg =
+              'The `npm ci` command can only install with an existing package-lock.json or\n' +
+              'npm-shrinkwrap.json with lockfileVersion >= 1. Run an install with npm@5 or\n' +
+              'later to generate a package-lock.json file, then try again.'
+        throw new Error(msg)
+      })
+    }
+
     await arb.reify(opts)
 
     if (!args.length && !isGlobalInstall && !ignoreScripts) {