site-install fails when site alias doesn't have defined root

[danepowell]

Describe the bug
When running drush si with an alias that has no root defined in Drush 9.6.2, you get an exception as described below. This seems to be related to a change in consolidation/site-alias, although I'm not clear on the full history: acquia/blt#3204

To Reproduce
Start from a vanilla Drupal codebase. Create a file drush/sites/foo.site.yml:

local:
  uri: foo

Now run drush @foo.local si.

(You don't actually need to set up sites.php or anything, since the site-install fails long before that.)

Expected behavior
Site to install.

Actual behavior

In SiteAliasTrait.php line 30:
Site alias @foo.local does not specify a root.

Workaround
Define a root in the alias file (which is specific to the local machine and not useful across teams). Use the workarounds described in acquia/blt#3204 (comment)

System Configuration

Q	A
Drush version?	9.6.2
Drupal version?	8.7.0
PHP version	7.2
OS?	Mac/Linux

Additional information
Using site aliases without a defined root is useful when you want to share local multisite aliases across teams for the purposes of installing a particular site (i.e. mostly just defining a uri).

[dpagini]

Sort of random note, but I found that in BLT's case (which passes site URI via drush config file), the VALUE of the root can be set to anything and the command will still work. It just needs it set to something. I found that a little odd.

[greg-1-anderson]

Pull requests welcome - I probably won't have time to get to this for a while.

[dpagini]

I'd love to help @greg-1-anderson but I now get a "talking to" at my employer for contributing code to the community.

¯_(ツ)_/¯ 🤦‍♂

[geek-merlin]

We also want this as we have aliases without root.

EDIT: We want to ssh to aliases without root.

[geek-merlin]

Trivial PR flying in: consolidation/site-alias#37

I can now do drush @foo ssh on such an alias.

EDIT: Said PR has a wrong approach.

[geek-merlin]

As clarified in the abandoned PR, the ssh use case has a different fix that this. So opened #4211.

[geek-merlin]

On site-install: TBH, i do not see where drush should install when no root is given.

[greg-1-anderson]

Presumably at the cwd.