Editing HIDE/Design/Project management (section)

=== Finding the package for a source file ===
There should also be a function to find the BuildInfo corresponding to a given source file. This is for cases like the interactive type checker. It needs to know what setting to use for any given source file. So it will use this function:

 ProjectManagement.getBuildInfoForFile :: FilePath -> IO (Maybe BuildInfo)

The project management component will figure out this mapping when the build tree root is set by reading the .cabal file(s) and figuring out which source files belong to it/them.

If a source file does not belong to any cabal package (for example if the user has not created any .cabal file or if it's merely some test module that is not part of the normal project build) then of course getBuildInfoForFile will return Nothing. In this case the interactive type checker would still be able to check the file but it would use default settings, as if the user were using ghc --make with no other flags.

Note that this allows us to extend things later for the case of more than one .cabal file in a build tree. Though it does make the (not unreasonable) assumption that each source file belongs to at most one cabal package.