refactor some nix-env and profile code to libcmd

Notably, ProfileManifest and ProfileElement are useful generic
profile management code, and nix profile is not the only place in the
codebase where profiles are relevant.

This commit is in preparation for fixing upgrade-nix's interaction with
new-style profiles.

Change-Id: Iefc8bbd34b4bc6012175cb3d6e6a8207973bc792

src/libcmd/command.hh

@@ -342,8 +342,6 @@ void completeFlakeRefWithFragment(
     const Strings & defaultFlakeAttrPaths,
     std::string_view prefix);
 
-std::string showVersions(const std::set<std::string> & versions);
-
 void printClosureDiff(
     ref<Store> store,
     const StorePath & beforePath,