Change DrupalInstaller locations to use contrib folder

[edysmp]

See #293

[jadb]

Please see: #285 (comment)

[edysmp]

@jadb Fixed unit test :)

[bojanz]

This makes sense. +1

[cweagans]

Sorry for the noise. Misunderstood what was happening in the test file. 👍 from me. This looks great.

[sbuzonas]

drupal-core, drupal-module, and drupal-theme were bold in the readme... Which it states they are stable and install paths will not be changed.

[typhonius]

👍 from me for this patch.

[rujiali]

Looks good to me.

[swim]

At time of writing the current composer file shipped with D8 core (8.6.3) defines the following installer paths.

"installer-paths": {
  "core": ["type:drupal-core"],
  "modules/contrib/{$name}": ["type:drupal-module"],
  "profiles/contrib/{$name}": ["type:drupal-profile"],
  "themes/contrib/{$name}": ["type:drupal-theme"],
  "drush/contrib/{$name}": ["type:drupal-drush"],
  "modules/custom/{$name}": ["type:drupal-custom-module"],
  "themes/custom/{$name}": ["type:drupal-custom-theme"]
}

The PR seems more aligned to the above 👍

[Seldaek]

Going to close this as I assume this is not needed anymore. If it is then it should be discussed in concert with the drupal team, as at this point so many Drupal sites are out there depending on the behavior here that I am not really comfortable changing the behavior without consulting people who are more in the know.