Merging duplicate namespace loaders

[franzliedke]

Some files can not be autoloaded when certain namespace prefixes are registered twice, but used for different classes.

Example, as reported by @tobscure:
flarum-auth-facebook requires league/oauth2-facebook, flarum-auth-github requires league/oauth2-github. Both of these packages specify autoload paths for League\OAuth2\Client\. but Studio only seems to merge in one of them:

'League\\OAuth2\\Client\\' => array($baseDir . '/extensions/flarum-auth-github/vendor/league/oauth2-client/src'),

[franzliedke]

For reference, October's implementation may be of help finding the right merging logic for each autoloading type.

[franzliedke]

Now not relevant anymore, since we don't manually merge these anymore, yay.