If composer.lock available, build full list of extensions for a PHP project `pie install`

[asgrim]

The initial implementation of pie install (when cwd is a PHP project) is to examine the root package and compile a list of extensions required. A more complete implementation should actually use composer.lock to compile the list of extensions required through transitive dependencies as well.

Example:

 - root package
   - requires: ext-foo
   - requires: a/b
     - requires: ext-baz

At the moment, ext-baz would NOT be picked for install (since we only look at the root package links).

Also note this comment from Stof:

It would be great to support identifying all extensions required by a project when a composer.lock file is present.

the heroku PHP buildpack has such logic looking at the lock file: https://github.com/heroku/heroku-buildpack-php/blob/6af5a55aab8a8f9417b8c8a14b0a6f91bf005aa0/bin/util/platform.php#L124

NOTE: this is work in progress already