Cardano Updates

* Avoid attempted rebuilding by `cabal`
* Eliminate the need for restoring cache
* Reduce the size of artifacts passed between `build` and `test`
* Reduce the fragile duplication of logic between `build` and `test`

doctesting has been removed but will be reinstated in a subsequent PR

Previously `projectConfigPragmas` read configure-args off the
first configured plan entry and emitted them under a single
`package *` block, on the assumption that "the same flags appear
in every configured plan entry's configure-args".  That holds for
project-wide toggles (--enable-optimization, --disable-shared,
...) but breaks the moment a user sets something per-package —
e.g. `package cabal-simple\n  profiling: True`.  The setting only
landed on cabal-simple's plan entry; if the first configured
entry was a transitive dep, the slice's cabal.project never saw
`profiling: True` and the exe built without `-prof`.

Group plan entries by `pkg-name`, take the union of pragmas
across each pkg's units (cabal.project only supports per-package
granularity), then split into:

  * `package *` — pragmas common to *every* pkg-name.  The slice's
    `cabal v2-build` resolves transitive hackage deps fresh, so
    these have to land project-wide for the deps' UnitIds to line
    up with plan-nix.

  * `package <name>` — for each pkg-name with pragmas extending
    the baseline (e.g. `package cabal-simple` getting
    `profiling: True` from
    `packages.cabal-simple.enableProfiling = true` mirrored into
    cabal.project).

`tests.cabal-simple-prof.run` now uses the per-package shape that
mirrors haskell.nix's modules:

  package *
    library-profiling: True
  package cabal-simple
    profiling: True