Replace `LANGUAGE CPP` with two platform-specific source files

`LANGUAGE CPP` works but comes with some downsides (surprising source
rewriting, more recompilations).  We can avoid them by having cabal deal with
the platform differences instead.

obelisk/frontend/android-src/FrontendPaths.hs

+-- | Implement platform-specific path-related operations for the Android
+-- platform.
+module FrontendPaths (getFilesDir) where
+
+import qualified Android.HaskellActivity as Android
+
+-- | Get the path to a directory that is private to the application where
+-- internal application data may be written (and read back).
+--
+-- The application requires no special permissions to use this directory.  If
+-- the app is uninstalled the directory and its children will be deleted.
+--
+-- https://developer.android.com/reference/android/content/Context#getFilesDir()
+getFilesDir :: IO (Maybe FilePath)
+getFilesDir = Android.getHaskellActivity >>= Android.getFilesDir

obelisk/frontend/frontend.cabal

@@ -3,6 +3,11 @@ version: 0.1
 cabal-version: >= 1.8
 build-type: Simple
 
+-- This ensures the platform-specific source files are also included in `cabal
+-- sdist`.
+extra-source-files: android-src/ActivityPath.hs
+                    linux-src/ActivityPath.hs
+
 library
   hs-source-dirs: src
   build-depends: base
@@ -17,8 +22,14 @@ library
                , bytestring
                , tahoe-chk
   if os(android)
+    -- Pull in the Android implementation of some platform-specific modules.
+    hs-source-dirs: android-src
+    -- And their dependencies.
     build-depends: android-activity
   else
+    -- Here, get the Linux implementation of those platform-specific modules.
+    hs-source-dirs: linux-src
+    -- And their dependencies.
     build-depends: xdg-basedir
     build-depends: directory
   exposed-modules:

obelisk/frontend/src/FrontendPaths.hs → obelisk/frontend/linux-src/FrontendPaths.hs

-{-# LANGUAGE CPP #-}
-
+-- | Implement platform-specific path-related operations for the Linux
+-- platform.
 module FrontendPaths (getFilesDir) where
 
-#if defined(linux_HOST_OS)
-
 import System.Environment.XDG.BaseDir (getUserDataDir)
 import System.Directory (createDirectoryIfMissing)
 
@@ -17,19 +15,3 @@ getFilesDir = do
 
   createDirectoryIfMissing True dataPath
   pure $ Just dataPath
-
-#else
-
-import qualified Android.HaskellActivity as Android
-
--- | Get the path to a directory that is private to the application where
--- internal application data may be written (and read back).
---
--- The application requires no special permissions to use this directory.  If
--- the app is uninstalled the directory and its children will be deleted.
---
--- https://developer.android.com/reference/android/content/Context#getFilesDir()
-getFilesDir :: IO (Maybe FilePath)
-getFilesDir = Android.getHaskellActivity >>= Android.getFilesDir
-
-#endif