Better support for plugins

[coot]

To configure a plugin one has to use ghc-options. This can be specified per component, e.g. see this example . The drawback of this is that cabal is passing these options not only when it compiles modules belonging to the package but it also runs:

/home/coot/.ghcup/bin/ghc \
  -hide-all-packages \
  -c /tmp/128418-0.c \
  -o /tmp/128418-1.o -hide-all-packages \
  '-plugin-package-id=ghc-tags-plugin-0.3.0.0-942a314601ea96d34f4332b9073d43cbdc47a8c68d4c8460bbac48721b5448d4' \
  '-fplugin=Plugin.GhcTags' \
  '-fplugin-opt=Plugin.GhcTags:../tags' \
  -fno-ignore-asserts

This command fails with an expected error:

<command line>: cannot satisfy -plugin-package-id ghc-tags-plugin-0.3.0.0-942a314601ea96d34f4332b9073d43cbdc47a8c68d4c8460bbac48721b5448d4
    (use -v for more information)
/home/coot/.ghcup/bin/ghc returned ExitFailure 1

The workaround is to also pass -package-db option. This option is not required when cabal actually compiles a package, and seems a bit strange at first, but makes sense when starts to analyse where it comes from.

cabal-install should either curate the options that it passes to ghc when it compiles the temporary .c file, or better it should have a proper way to configure plugins. Ideally, it would be a package level configuration as well as a global option, which would allow to configure a plugin globally and allow to set plugin options per package, or allow to configure different plugins for different packages.