Move value-only methods to `InstallableValue`

These methods would previously fail on the other `Installable`s, so
moving them to this class is more correct as to where they actually
work.

Additionally, a `InstallableValueCommand` is created to make it easier
(or rather no worse than before) to write commands that just work on
`InstallableValue`s.

Besides being a cleanup to avoid failing default methods, this gets us
closer to https://github.com/NixOS/rfcs/pull/134.

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