chore: move executable code to src/

src/ now contains all source code - in subdirectories, according to the
.cabal component they belong to. This will allow us to put vendored
libraries in the same place - and later split our own code into multiple
components/libraries as well.

postgrest.cabal

@@ -42,7 +42,7 @@ library
   default-extensions: OverloadedStrings
                       NoImplicitPrelude
                       NumericUnderscores
-  hs-source-dirs:     src
+  hs-source-dirs:     src/library
   exposed-modules:    PostgREST.Admin
                       PostgREST.App
                       PostgREST.AppState
@@ -194,7 +194,7 @@ executable postgrest
   default-language:   Haskell2010
   default-extensions: OverloadedStrings
                       NoImplicitPrelude
-  hs-source-dirs:     main
+  hs-source-dirs:     src/executable
   main-is:            Main.hs
   build-depends:      base                >= 4.9 && < 4.22
                     , containers          >= 0.5.7 && < 0.8