This command takes a list of ports, and causes the final state of the
installed directory to be as-if one ran the install on an empty
installed directory (removing any unnecessary packages).
This is especially useful with the new `--x-install-root` option, which
allows one to set the `installed` directory for vcpkg to use.
Additionally, as a drive-by, we do some `stdfs` clean-up and add a
`.is_feature()` member function to BinaryParagraph (as opposed to
checking for `.feature().empty()`, which is far less clear to read).
This feature is experimental.
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