foundationdb: refactor 'vsmake' build system into its own file [NFC]

FoundationDB is currently in the process of migrating to CMake, and it
will eventually be the only build system. In preparation for this, split
off the current (somewhat nasty) builder into its own file, and allow
default.nix to be more declarative -- containing only the main supported
versions.

Similarly, a cmake.nix file will be added later.

There is no functional change here (NFC), only an organizational change
(file moves, no hash changes).

Signed-off-by: Austin Seipp <aseipp@pobox.com>