migrations library: fix broken install() method

install() wasn't updated to match the new directory layout.  Also it's
redundant with the get_latest_version() method.  The extra filtering in
install() is interesting, but being inconsistent about that is obviously a
Bad Idea.

Fix get_latest_version() to use the stricter filtering, (but still use
the correct paths).  Then fix install() so it just calls
get_latest_version().
Migrations library: tidyup

 - Move related methods a bit closer together
 - Trivial refactor of auto_latest() to use install()
 - Nitpick comment consistency.
migrations library: remove set_path() method

The migrations controller used set_path() for module migrations, but that
wasn't needed, at least not any more.

The problem with setting the path is there are actually three different
paths.  (This is more obvious following the new directory structure, but
I think the same problem applied before as well).
install: update Migrations library

From newest to oldest, I see
 - Language lines moved to application_lang.php
 - Recent removal of unused config options
 - Fix name of 'migrations_path' param
   (and update the callsite in Installer_lib.php).
 - Use the "new" layout of the schema_version table
   (see 012_Migration_schema_change.php)