Remove timestamp based rebuilds (#1174)

If you want to build a package without changing the version number,
please use `--force` from now on. For example:

    pmbootstrap build --force hello-world

Prior to this commit, changes were detected automatically (timestamp
based rebuilds). However, that feature does not work as expected with
the binary package repository we have now, and depending on how you use
git, it has never worked. Close #1167, close #1156, close #1023 and
close #985. This commit also mentions --force when a package is up to date,
but the user requested to build it.