Leverage wikimedia/composer-merge-plugin

Architecturally I think 8.x-1.x could benefit the quickest from stealing a few tricks from wikimedia/composer-merge-plugin (registering a plugin instead of having custom drupal-* commands, resolving duplicates).

Probably not something I'll have time to explore right now, since the current solution works (and the fact that composer.json is actually overwritten instead of dynamically modified means that it's very debuggable).

Worth noting that core itself may leverage the composer-merge plugin. See attached issue

And by attached issue I mean this one:

The core patch has landed.

I've committed the first two cleanups based on that:
http://cgit.drupalcode.org/composer_manager/commit/?id=138b22f
http://cgit.drupalcode.org/composer_manager/commit/?id=4a57f13
In total 500 lines of code removed.

This issue can now be attempted (adding modules to the replace and merge fields instead of adding their dependencies to require and require-dev).

[8.x-1.x e5a7165] Issue #2444459 by bojanz: Leverage wikimedia/composer-merge-plugin
 1 file changed, 24 insertions(+), 24 deletions(-)

That was easy :)