python3Modules.meson-python: prepare for structuredAttrs

[SFrijters]

Eventually this variable should be quoted, but this is backwards compatible for now.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[uninsane]

this hook borrows from pkgs/by-name/me/meson/setup-hook.sh; the meson hook itself was made compatible with structuredAttrs here.

i don't know enough about structuredAttrs to say if your approach here is "correct" or not. my intuition would be to track the original meson setup-hook as close as possible though, i.e.

local mesonFlags=()
concatTo flagsArray mesonFlags mesonFlagsArray

tagging @doronbehar for your familiarity in these areas

[doronbehar]

I'm too not familiar enough with structuredAttrs related changes and I don't understand how this change or @Unisane's suggestion prepares for structuredAttrs. However, @Unisane's approach feels slightly bit better.