Use the project template composer.json to set up Drupal CMS in DDEV

Right now, we have a composer.json at the project root (which is just used for internal development) and a project_template/composer.json, which would be used as the basis for real projects. These two are nearly identical, except that the one at the project root explicitly lists the components' path repositories, and requires dev dependencies. This duplication feels wrong to me, and it leads to us needing a more complicated CI pipeline.

Let's change the build process in such a way that we're only using the project template as the basis for local development and CI. The ddev drupal-install script should take care of preparing the root composer.json, by copying and modifying the project template. Our CI process, meanwhile, should use composer create-project to create a code base using the project template, and then set it up (ideally by reusing the DDEV scripts) to run tests.