Use Keruald\Commands package to provide Command class

Summary:
The Command, Display and OutputDisplay class have been extracted
to the Keruald/Commands library in rKCMD21b2d7f16ee9.

This change removes the extracted files and declare the library
as a dependency.

Ref T1386.

Test Plan:

• bin/saas-mediawiki-get-host.php
• bin/saas-mediawiki-get-host.php nonexisting
• bin/saas-mediawiki-get-host.php utopia

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1386

Differential Revision: https://devcentral.nasqueron.org/D1555