lib: Implement 'only' that creates a dependency chain from stdin

When building closed loops of packages you sometime don't want the
entire chain to be resolved to just figure out the order of 5 packages.
This implement `--only` which resolves the chain and ensures only the
packages from stdin are included in the output chain.

This is useful for rebuild lists or when building tight loops of
packages.

    $ arch-rebuild-order -o cmake meson jsoncpp
    meson jsoncpp cmake
    $ arch-rebuild-order -o jsoncpp meson cmake
    meson jsoncpp cmake

Signed-off-by: Morten Linderud <morten@linderud.pw>
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