refactor(grfn/xanthous): avoid unnecessary recompilation

hpack is a bit dumb when generating the list of modules for a cabal
file's component if multiple of them live in the same directory.
Specifically it seems to assume that all modules in the source-dirs
of a particular component are also necessary for its compilation.

This is quite bad in the case of xanthous since both library and
executable have source-dirs: src, so all modules will be compiled
twice: Once for the library and then again for the executable
despite it depending on the library (actually 4 times in total
since we need to build a unprofiled and profiled object for each
module…).

To fix this we just move Main.hs into its own directory and change
the executable's source-dirs, so hpack doesn't get confused anymore.
Since all components now have their own source-dirs, unnecessary
redundant compilation should be down to 0. The diff of the cabal
file shows quite nicely how many module recompilation we've gotten
rid of.

Change-Id: I2df4fab9b0299b3a2b5d3005508c79b2d9796039
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3533
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>

users/grfn/xanthous/package.yaml

@@ -116,7 +116,7 @@ library:
   source-dirs: src
 
 executable:
-  source-dirs: src
+  source-dirs: app
   main: Main.hs
   dependencies:
   - xanthous