Finalize DI for sitewide command packages

[geek-merlin]

Urgently needed this. Found a code comment how to do it. Did it.

[geek-merlin]

Patch worksforme. Only thing left is to fix the path expression.

[weitzman]

Needs a test and code style fixes.

[geek-merlin]

OK Codestyle is fixed now.

Next i can use some hints:

        // @fixme Fix directory to use project root.
        $this->addDrushServiceProviderFromSubdirectories(dirname(DRUPAL_ROOT) . '/drush');

Deriving the drush dir from Drupal root is wrong but the best i could figure out. I think it should be derived from project root.

@weitzman What is the canonical way to get the project root?

(Also memo to me: Some tests fail with windows-style paths. So use DIRECTORY_SEPARATOR, not '/')

@weitzman As of tests: Not my strong point, but i'll try. Can you give me a hint how to approach this?

[weitzman]

I',m not sure the answer to those questions. Needs a review. Maybe @greg-1-anderson or @claudiu-cristea can review.

[weitzman]

This brings complexity. if anyone really wants it please post a use case.