Check for invalid depends_on() in migrations

[iMattPro]

EPV should check that depends_on() array's contain strings that always start with a slash.

So this is valid:

static public function depends_on()
{
    return array('\foo\bar', '\bar\foo');
}

And we should error for any missing leading slashes:

static public function depends_on()
{
    return array('foo\bar', '\bar\foo');
}

[avandenberghe]

👍