Address issues noted while writing Haskell platform system

[ndmitchell]

@mzero kept an awesome log of his running thoughts while using Shake, available here: https://github.com/haskell/haskell-platform/blob/new-build/notes/BUILD-NEW#L276

I should seek clarification on a few of them, fix a few others, improve docs in other areas and generally try and reduce the list to nothing.

(Note to all: keeping a log of your ongoing frustrations as you hit them is awesome, if anyone else has such a log for Shake, no matter how misguided the contents might seem on reflection, please send it over)

[bergey]

Notes are now at https://github.com/haskell/haskell-platform/blob/master/notes/BUILD-NEW#L273

[ndmitchell]

Thanks @bergey. Mirroring them here:

[] building dirs needs better support
[] when you build something, you must build it afresh!
    [] and then never modify it
    [] these requirements come to light when building dirs
[] global Rules() make it hard to be modular
    [] if package A has rules for type foo
    and package B & C need foos, then both can't call the function
    in A that provides the rules

[] because need takes a list, and you only get parallelism if you
    supply all needs at once, it is hard to modularlize targets
    that are based on some hidden info

[] addPath isn't useful because command and friends use createProcess
    which looks up the executable in the current environment, not the
    modified environment that will be accompany the final execution

[] copyFile' should putNormal?

[] staunch seems to give up early?

[] would love a command line option: just print the known targets and oracles
[] would love an command line option that prints an explaination for each thing

[] would be nice if there was
    knowsHow :: FilePath -> Rule Bool