bin/mdw-build: Don't assign `$srcpath' until after `checkout'.

Otherwise we end up printing two `--- srcpath = ...' lines in the common
`checkout' case, which is just silly.

Instead:

  * set `$srcpath' in the `checkout' stanza, either pointing the new
    tree or back at the original one; and

  * use `$topath' for the original source tree until we've established
    `$srcpath'.