Post install messages should be in ghcup-x.x.x.yaml

This allows easier updating of instructions, e.g. for HLS (background: current ghc version needs to be set to the ghc project version, see https://github.com/mpickering/hie-bios/issues/194).

Open questions:

1. should post install messages be per tool, tool+arch or tool+arch+version?
2. do we also want post-remove?

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Implementation guide:

• this could be added to toolRequirements, which we should probably rename (and have a backwards-compatible json instance). This is currently per tool -> toolVersion -> distro -> distro version, while we use unknown_version/unknown_versioning for toolVersion/distro version, see ghcup-0.0.4.yaml
• alternatively, add a new top-level dictionary (will also make backwards compat easier)?
• we then need to collect those post-install messages in Main.hs and print them instead of the hard-coded messages
• all in all the result yaml should still be parsable by older ghcups