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Try to depend on imported derivations

Sometimes we'll import the result of a derivation, e.g. a Haskell
package might come from the result of runCabal2nix, and that result may
have depended on a mechanism like haskellPkgDeps to pick the versions,
and that might depend on stableHackageDb which takes *ages* to build.

The problem is, if we have such a package installed and we do a garbage
collection, we'll end up deleting those derivations that we imported, so
rebuilding the installed package (e.g. during 'nixos-rebuild switch')
will end up rebuilding those to-be-imported derivations; *even if the
installed package doesn't actually change*!

We could try to solve this "locally", by making the thing we import from
a derivation itself depend on that derivation. For example, if we're
making a Haskell package from a runCabal2nix derivation, we could add
that runCabal2nix derivation as a dependency of the Haskell package
using 'withDeps'.

The trouble with doing that is the possibility of causing unnecessary
rebuilds. For example, let's say we bump stableHackageDb: that causes
the runCabal2nix derivation to change (since it has a different input),
which (due to 'withDeps') would cause the Haskell package to get
rebuild: *even if the resulting Haskell package doesn't change*!

One of the nice features of importing from a derivation is that we can
sever the dependency chain: if derivations foo and bar both result in
equivalent Nix expressions, say for baz, then switching out foo with bar
won't cause baz to get rebuilt.

I think the best solution is to depend on such derivations in "combo"
packages, i.e. those which only exist to expose some binary, or merge
together the contents of other packages. Rebuilding such packages is
fine, since they're just symlink farms, and if they don't have many
things depending on them (apart from other link farms) then cascading
rebuilds aren't too bad.

We could, for example, use 'pandocPkgs' for this.

To facilitate such use, functions which rely on importing expensive
derivations should pass along those derivations in their output, so that
their callers can either add them as dependencies, or pass them along in
turn. I'm mostly thinking about the haskellPkgDeps functions here.