Simplified site aliases. Fixes #3107

[greg-1-anderson]

First pass. Mostly done.

• Ad-hoc usage is pretty consistent
• Isolation tests are passing
• Functional test fixtures (e.g. sut aliases) may still need adjusting
• There may be a little more site alias code that can be deleted

[greg-1-anderson]

Isolation tests are failing because a list needs to be sorted before the assertion.

[weitzman]

Looks like this dir is now 'sites', not 'site-aliases'.

[greg-1-anderson]

I increased the search depth of site aliases to <=1 so that it would be possible to store related alias files together in a directory (e.g. ~/.drush/sites/pantheon/example.site.yml).

Currently, the behavior of the site:alias command is as follows:

Command	Result
drush sa	Show all sites from all site alias files that can be found
drush sa @example.dev	Show just the dev environment of the example site
drush sa @example	Show all of the environments of the example site

In the last example, environments defined in the file self.site.yml are also included for the first local site found in example.site.yml.

The next question is, should there be some way to run drush sa with an additional argument or option to show all of your aliases in a particular directory (e.g. all of your aliases for a certain isp)?

drush sa --group=dirname
drush sa --filter=pathregex
drush sa /regex/

Or perhaps some other notation?

[greg-1-anderson]

All green! We could merge here and solve ^^ as a follow-on issue, or fix it up prior to merging as desired.

[weitzman]

I don't feel the need to see aliases in a particular directory. Thats getting real close to `more /path/to/site.yml`. So, lets merge this now.