Findable extra documentation

[bos]

(Imported from Trac #674, reported by jbapple on 2010-05-01)

I have a cabal package that should include extra documentation in the distribution. This documentation should not be written in Haddock.

1. There should be a field for this analogous to data-files or extra-source-files. The former is too much: the files do not need to be accessed by any Haskell runtime. The latter is not enough: it does not store the extracted files on the system on which the package is installed.
2. There should be a way for humans to find these files without loading up a GHC runtime or using Unix's "find". I suggest "cabal list" also shows, for installed packages with extra data or documentation files installed, where these files can be found.

[bos]

(Imported comment by @dcoutts on 2010-05-01)

See ticket #330.

[bos]

(Imported comment by @dcoutts on 2010-05-03)

Note for point 2, this relies on us knowing if documentation files are installed and if so where.

Currently cabal info pkgname will report the location of documentation if the haddock-html field points to a directory that exists. This is only appropriate for html documentation.

One option to support more general documentation would be via cabal tracking files for packages it installs, however that would not help for packages installed with ghc or via the system package manager.

Another thing to note here is that the fact that we track whether or not haddock docs are installed via ghc-pkg. This itself is a bit of an oddity. It is not information that ghc itself needs and nor is it package meta-data (like the author or description). It's just auxiliary information about aspects of the installed package. It's sort of package management information. It's not clear this special case makes sense. Probably we should allow more general package information (e.g. location of various kinds of docs) or we should not track haddock information in the ghc-pkg registration information at all.

[bos]

(Imported comment by @dcoutts on 2010-06-18)

See also #710 about license files specifically.

[quasicomputational]

extra-doc-files is now a thing, which ought to satisfy this issue.