This hook is called when an extension is installed.
To be specific, this hook is called when an extension's status changes from uninstalled to enabled
It is NOT called when an extension moves from disabled to enabled
hook_civicrm_enable is called at this event). Each module will
hook_civicrm_install during its own installation (but not
during the installation of unrelated modules).
For more background, see API and the Art of Installation.