Fixed sf43 deprecations for future compatibility

[linaori]

When upgrading to 4.3 I got a bunch of deprecations, this PR should fix them.

Summary:

• Fixed the root node access/creation in the Configuration
• Commands no longer extend container aware

One remaining issue is that the InstallFontCommand is hardcoded to use the web directory, which is configurable in Symfony and defaults to public, but was outside of my scope.

Running phpunit gave me a bunch of risky tests as no assertions were made, so I've tested the changes in my own application:

• ran the 3 commands with --help to see if their initialization works
• compiled the container to see if the deprecations were gone

[deguif]

Any news on this? Symfony 5 is approaching ;)

[deguif]

Ping @isometriks as this is currently critical. Symfony5 was released nearly a year ago ;)

[deguif]

That should now be addressed by #1256 and #1249