Cardano Updates

Remove inheritance from `shell.nix` when defining `hsPkgs`. I tried following the tutorial on a new project and I had errors caused by this.

`cabal configure` for some large haskell projects (such as
`cardano-node`) can take several minutes to run.

When building locally `cleanSourceWith` is used so that only changes to
`cabal.project` and `.cabal` cause `cabal configure` to be run again.

This does not work on hydra though and every commit will cause
`cabal configure` to be rerun during `eval`.  It also means that
local builds cannot reuse the results from hydra.

To fix this `copyTextDir` runs `__readFile` and `writeTextFile` on
the filtered subset of files (cabal.project and .cabal).

Even if the input path is a different store directory the resulting
derivation will be the same if the contents of the filtered subset
of files is the same.

Makes sure "/" character is added when needed.

The legacyPackages only gives access to the default nixpkgs version, not
any of the others.

With this change it's possible to select the nixpkgs one wants to use
with:

  pkgs = import haskell-nix.sources.nixpkgs-2003 {
    config = import (haskell-nix + "/config.nix");
    overlays = [ haskell-nix.overlay ];
    localSystem.system = system;
  };

The [dependent-map](https://hackage.haskell.org/package/dependent-map)
package for instance is not clear on which free license applies to
which parts of the code, but that does not mean it is not free.

`cabal2nix` maps LicenseRef-OtherLicense to `meta.license = "unknown"`

`cabal-to-nix` and `haskell.nix` map it to a more detailed attribute
set but it includes `free = false` and that causes packages like
`dependent-map` to fail to install when `allowUnfree` has not been set.

This change removes the `free` attribute altogether, which seems more
principled than asserting that the `OtherLicense` cannot be `free`.

Makes sure "/" character is added when needed.

The [dependent-map](https://hackage.haskell.org/package/dependent-map)
package for instance is not clear on which free license applies to
which parts of the code, but that does not mean it is not free.

`cabal2nix` maps LicenseRef-OtherLicense to `meta.license = "unknown"`

`cabal-to-nix` and `haskell.nix` map it to a more detailed attribute
set but it includes `free = false` and that causes packages like
`dependent-map` to fail to install when `allowUnfree` has not been set.

This change removes the `free` attribute altogether, which seems more
principled than asserting that the `OtherLicense` cannot be `free`.

When running a test we should do it in the same sub directory the
test is built in.