sql:sync does not accept a directory as a target

[kevinquillen]

Describe the bug
Using sql:sync, the command docs state:

target* : A site-alias or the name of a subdirectory within /sites whose database you want to replace.

However when passing a directory as an argument instead of an alias, it errors out.

To Reproduce
drush sql:sync @remote.source 'directory name in /sites'

Expected behavior
SQL sync / drush would pick up the db connection info in the provided sites directory.

Actual behavior
Received an error about no site alias found for 'directory name'.