Note [Base Dir] ~~~~~~~~~~~~~~~~~

GHC's base directory or top directory containers miscellaneous settings and the package database. The main compiler of course needs this directory to read those settings and read and write packages. ghc-pkg uses it to find the global package database too.

In the interest of making GHC builds more relocatable, many settings also will expand `${top_dir}` inside strings so GHC doesn't need to know it's on installation location at build time. ghc-pkg also can expand those variables and so needs the top dir location to do that too.