meta/pkg-build: fix potential problem with cycle detection

Also, correct typos and out-of-date information in comments that
serve as documentation.
meta/pkg-build: add support for "minimal" vs. full VMs

Try building on a minimal VM, first, and then fall back to a more
general VM is that fails. For example, a minimal VM might not include
a C compiler, which can help clarify that a package will work in some
environments and not in others.
meta/pkg-build: run `raco test --drdr` for each package
meta/pkg-build: always set the VM clock

Consistent installation depends on a global clock that always
moves forward, so make sure VMs stay in sync.