Clean up file paths for new module hierarchy
This updates comments only.
This patch replaces file references according to new module hierarchy [1][2].
For files under the compiler/
directory, I replace them as
module paths instead of file paths. For instance,
GHC.Unit.State
instead of compiler/GHC/Unit/State.hs
[3].
For current and future haddock's markup, this patch encloses
the module name with ""
[4].