remove no_vdb restriction if --preload-vdb-state given

[4eetah]

Fixes #51

[codecov-io]

Current coverage is 55.23% (diff: 0.00%)

Merging #218 into master will decrease coverage by <.01%

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[4eetah]

This is actually a quick fix which implicitly imposes --noreplace option.
Currently, things work in a way that if no --noreplace option is given the additional restriction is added (_vdb_restrict) so that resolver doesn't look through livefs_vdb and virtual packages.
I guess the more properly would be to still allow for rebuild of already installed pkgs even though --preload-vdb-state is given, which seems inevitably trigger a lot more changes.