Replace hook_drush_load() with --exclude

I don't think we can get away from foo.drush.load.inc right now, since --ignored-modules only appears to be taken into account on DRUSH_BOOTSTRAP_DRUPAL_FULL. But we need to block provision (drush) commandfiles from being loaded. I guess we'd need something like an '--exclude' option (the opposite of '--include').

I'm going to close this issue for now. If Drush introduces a '--exclude' (or similar) option, we can revisit it.

Moshe just pointed out a good place to start: drush_filename_blacklist()

I'll write up a patch to add an '--exclude' option that gets tacked onto the $blacklist array.

See: https://github.com/drush-ops/drush/pull/437

https://github.com/drush-ops/drush/pull/437 was committed to to Drush in b1e3082. This removed support for *.drush.load.inc files.

We'll need to adapt our hosting modules registry accordingly.

I added hosting_feature_enabled($feature) to allow for easily checking whether a Hosting feature is enabled. Fixed hosting in ec58467.

I removed all calls to drush_load_subdirs() from Provision, so Aegir should work on Drush 7 again. Fixed provision in 4f11e69.

This is fixed now, and Aegir will install at the commit in Drush where this broke things. However, some commit to Drush since is blocking installation. See #2281983: Support installation of Aegir on Drush 7.x for follow-up.