Originally the virtual packages could have dependencies added to it.
However, commit b06e3b99 broke this behaviour to fix error reporting.
The root cause however was that the virtual depedency package was not
properly versioned.

This fixes to use current date/time as the package version, and
constructs the "faked" package hash from it. This effectively makes
"add -t virtpkg deps.." replace the dependencies which should be the
desired behaviour for "abuild deps".

'world' dependency to the generated virtual package is also now
versioned to make sure it get's upgraded.