Replace git submodules with Composer dependencies

Here's the current picture:

* when loading runtime depenencies, wp-cli first looks in the `vendor` folder and then falls back to the git submodule paths.

* when building the Phar archive, only git submodules are supported.

* when running tests, only Composer dependencies are supported.

This is rather inelegant, since the dependency definitions have to be defined twice and duplication always leads to problems.

The alternatives are:

A) Ditch Composer dependencies:

* remove `composer.json`
* have people manually create a `composer.json` file, so that they can install Behat
* let people figure out how to install PHPUnit on their own

B) Ditch git submodules:

* [ ] make `utils/dev-build` install Composer and run `composer install`
* [ ] un-ignore `composer.lock`, so that everyones get the exact same versions of things
* [ ] make the Phar building script look in the `vendor` directory

Since A) seems like a step backwards, I'm going to take a stab at B).