Improve abstraction

[ndmitchell]

Think about and potentially address the issues raised at https://gist.github.com/Profpatsch/03f97fd8b6f73d5fa279dc869b44844f by @Profpatsch

[ndmitchell]

3 issues in the above gist:

1. Setting a default out directory

      -- how to express on a non-string level that everything should only ever be put into "_build"?
      -- how to never repeat that information anywhere else, much less do problematic string operations
      -- with the `"_build"` constant?
      staticDir = "static"
      staticFiles = map (staticDir <>) [ "foo" "bar" "baz" ]

In that case probably redefining %> etc. to prepend _build probably works. Unfortunately I don't think there's any magic bullet, since if you are running command lines they might leak outputs somewhere.

2. Pulling apart a file pattern

  (buildDir </> staticDir </> "*") %> \outFile ->
    -- we only get `out`, which is some unknown string
    -- so: how to recover the semantic information where to find the static file?
    let inFile = magicallyAndManuallyStriptheBuildDirAgain outFile
    copyFile inFile outFile

This has been discussed before with #257, and a new function was added to make it possible in #499. Things are now being moved out to a separate filepattern library, so there's scope here - the only problem is getting type safety and construction from a string.

3. Clean dangers

  -- another thing:
  -- No separation between build inputs and mutated files
  phony "clean, maybe?" $ do
    removeFiles "foo"
    -- oops, of course should have been `buildDir </> "foo"`.
    -- ah, well, who needed those source files anyway.

They are separated internally, so I guess #273 might be the way to go, and then cleaning isn't so dangerous.