Cardano Updates

* make-install-plan: Include info needed for backpack.

* Support backpack builds.

Fixes #244

* fixup! Support backpack builds.

* Bump head.hackage

* Dump head.hackage

* Work around GHC JS backend not producing .o files for backpack signatures

GHC's JavaScript backend doesn't produce .o files for backpack signature
module instantiations, causing ar to fail when creating library archives.

Fix this by replacing the ar command in GHC's settings with a wrapper
that filters out missing .o files from both command-line arguments and
response files. The fix only applies to GHCJS instantiated backpack
builds, leaving all other builds unchanged.

---------

Co-authored-by: Hamish Mackenzie <[email protected]>

# Conflicts:
#	test/cabal.project.local

* fix: replace builtins.appendContext with context-carrying concatenation

builtins.appendContext validates that store paths exist locally, which
fails when paths come from flake inputs that aren't available on the
evaluating machine (e.g. CHaP in cardano-node builds).

Instead, preserve string context through parsing by using
builtins.substring 0 0 to capture context from input strings and
re-attach it via concatenation after builtins.match/split strip it.

- parseBlockLines: carry context from input lines to parsed attr values
- parseSourceRepositoryPackages: carry context through builtins.split
- extractSourceRepoPackageData: use unsafeDiscardStringContext for
  sha256map attribute name lookups
- parseRepositoryBlock: remove addContext helper that used appendContext,
  use attrs.url directly (context now preserved by parseBlockLines)
- call-cabal-project-to-nix: remove appendContext on writeText, export
  rawCabalProjectContext for use by load-cabal-plan
- load-cabal-plan: use rawCabalProjectContext concatenation instead of
  appendContext

Add unit tests verifying context flows through parseBlockLines and
parseSourceRepositoryPackages.

* Bump head.hackage

GHC's JavaScript backend doesn't produce .o files for backpack signature
module instantiations, causing ar to fail when creating library archives.

Fix this by replacing the ar command in GHC's settings with a wrapper
that filters out missing .o files from both command-line arguments and
response files. The fix only applies to GHCJS instantiated backpack
builds, leaving all other builds unchanged.