Use Map with custom FromJSON to ignore unknown keys

[dfordivam]

Normally the ghcup would complain for parse failures in Platform and Architecture.
With this PR these will be ignored, and parse would succeed

[ Error ] [GHCup-00160] JSON decoding failed with: Aeson exception:
[ ...   ] Error in $.toolRequirements.GHC['unknown_version'].Darwin2: Failure in Platform (FromJSONKey)

[ Error ] [GHCup-00160] JSON decoding failed with: Aeson exception:
[ ...   ] Error in $.ghcupDownloads.GHC['8.2.2'].viArch['A_64_2']: invalid key "A_64_2", expected one of ["A_64","A_32","A_PowerPC","A_PowerPC64","A_Sparc","A_Sparc64","A_ARM","A_ARM64"]

[hasufell]

can we not have instances for type aliases?