Rewrite bootstrap.sh

[phadej]

It should have:

• hardcoded lists of dependencies per GHC (and platform)
• bail out for unsupported combinations

These lists should be autogenerated, with help of cabal-plan (I have done something close in spirit in https://github.com/phadej/cabal-extras/tree/master/cabal-bundler so I know it's possible).

It should also check the hashes of .tar.gz and revision .cabal files downloaded (cabal-bundler does that).

Why so? Because bootstrap.sh is largely not maintained. We test it with single ghc-compiler version. Much better would be if it was "correct by construction".

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The script should always work with self-created ghc-pkg init package database (which is called sandbox in current script).

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It should not generate documentation, when there's working cabal-install, one could iterate to get it. Same applies for anything extra. bootstrap.sh purpose is to get first working version on platform where there are no (older) cabal-install. All other use-cases are out-of-scope.

[bgamari]

I would imagine it would also be helpful to write it in a language that isn't bash. This is the sort of thing I usually fall back onto Python for.

[bgamari]

I gave this a quick shot in #6719.

[phadej]

Continued in #6979