'drupal-custom-module' in template needs to be prefixed with 'type:'

[mariagwyn]

The BLT composer template incorrectly writes the custom module installer path.

Current:
"docroot/modules/custom/{$name}": [ "drupal-custom-module" ],
Should be:
"docroot/modules/custom/{$name}": [ "type:drupal-custom-module" ],

Without this change, custom modules of this type are installed in /modules/custom/$name rather than /docroot/modules/custom/$name

[mariagwyn]

Ok, pretty sure I should have just filed the PR, but this is fixed with #1085.