clarify absPathFrom

The repo path is typically relative, not absolute, so
providing it to absPathFrom doesn't yield an absolute path.
This is not a bug, just unclear documentation.

Indeed, there seem to be no reason to simplifyPath here, which absPathFrom
does, so instead just combine the repo path and the TopFilePath.

Also, removed an export of the TopFilePath constructor; asTopFilePath
is provided to construct one as-is.

Utility/Path.hs

@@ -60,7 +60,7 @@ simplifyPath path = dropTrailingPathSeparator $
 {- Makes a path absolute.
  -
  - The first parameter is a base directory (ie, the cwd) to use if the path
- - is not already absolute.
+ - is not already absolute, and should itsef be absolute.
  -
  - Does not attempt to deal with edge cases or ensure security with
  - untrusted inputs.