SysTools: Expand occurrences of $topdir anywhere in a Settings path
SysTools: Expand occurrences of $topdir anywhere in a Settings path

Subscribers: rwbarton, thomie, Phyx

Differential Revision: https://phabricator.haskell.org/D4221

sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc