Create ghcup distribution

[hasufell]

TODO

• Support installing symlinks via installTheSpec haskell/ghcup-hs#1351
• Use gnu tar on FreeBSD #182

Example metadata:

toolRequirements: {}
ghcupDownloads:
  stable-ghc:
    toolDetails:
      toolHomepage: "https://stable-haskell.github.io/ghc/"
      toolRepository: "https://github.com/stable-haskell/ghc"
      toolDescription: "The stable Haskell GHC fork"
      toolLicense: "BSD-3"
    toolVersions:
      9.14:
        viTags:
        - base-4.22.0.0
        - old
        viArch:
          A_64:
            FreeBSD:
              unknown_versioning:
                dlUri: file:///home/hasufell/git/ghc/_build/dist/ghc.tar.gz
                dlHash: c2a2404e304eb647b817bdca183bc815e8f1fecf8ce289abc6b9358195f54912
                dlInstallSpec:
                  exeRules:
                    - installPattern: ["bin/*"]
                  dataRules:
                    - installPattern: ["lib/**"]
                  exeSymLinked:
                  - linkName: "stable-${TARGETFN}-${PKGVER}"
                    pVPMajorLinks: false
                    setName: "stable-${TARGETFN}"
                    targetPattern: ["bin/*"]
                  preserveMtimes: true