Allow _defaults declaration under services

[iMattPro]

Some extension developers are starting to use Autowiring. That requires a _defaults declaration under services:

services:
    _defaults:
        autowire: true

This change will serve to allow that, and prevent the following false error:

Warning: The service name should start with vendor.namespace (which is vendorname.extensionname) but started
with _defaults in /vendorname/extensionname/config/services.yml

Also there are some additional code improvements and error message corrections in this test.