vala: search for vapi files in targetOffset, not hostOffset

[uninsane]

Description of changes

this is in response to an old TODO: #189397 (comment); CC @Artturin

this PR should only change things (practically) for cross-compiled builds. previously, vala projects would struggle to find the vapi (vala API) files of their build dependencies. workarounds were to copy these dependencies into nativeBuildInputs, or provide the project some other way to find the API info (e.g. gobject-introspection). vala now looks in the right place for vapi files, as demonstrated by the commits in this patch which remove both class of workaround.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

Priorities

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[Artturin]

I opened a pr doing the same thing last week #267550

[uninsane]

I opened a pr doing the same thing last week #267550

nice, feel free to CC me in the future if it's work spawned from a PR discussion like that btw