refactor: Remove dependency on Paths_ module

The cabal-provided Paths_ module allows us to use the version number
from postgrest.cabal. This can be done equally well with the GHC-defined
CPP macro "VERSION_postgrest".

By making this change we avoid the inclusion of the Paths_ module, which
also stores some paths related to the cabal configuration. Those are
problematic to go into the final executable, because for nix-based
builds those are paths to the /nix/store/... - which means that our
static executable then depends on those paths.. and we can't build a
minimal docker image anymore.

To counter this, we have been using dead code elimination when building
the static executable. This has been working well, but there is a
problem on aarch64-darwin, which we will hit once can finally make our
way there: GHC on aarch64-darwin (or darwin in general?) can't do dead
code elimination - and thus it'd be impossible to create those minimal
docker images for those platforms. More information upstream in nixpkgs:
https://github.com/NixOS/nixpkgs/issues/318013

src/PostgREST/Version.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP             #-}
 {-# LANGUAGE TemplateHaskell #-}
 module PostgREST.Version
   ( docsVersion
@@ -7,19 +8,19 @@ module PostgREST.Version
 import qualified Data.ByteString as BS
 import qualified Data.Text       as T
 
-import Data.Version       (showVersion, versionBranch)
 import Development.GitRev (gitHash)
-import Paths_postgrest    (version)
 
 import Protolude
 
+version :: [Text]
+version = T.splitOn "." VERSION_postgrest
 
 -- | User friendly version number such as '1.1.1'.
 -- Pre-release versions are tagged as such, e.g., '1.1 (pre-release)'.
 -- If a git hash is available, it's added to the version, e.g., '1.1.1 (abcdef0)'.
 prettyVersion :: ByteString
 prettyVersion =
-  toUtf8 (showVersion version) <> preRelease <> gitRev
+  VERSION_postgrest <> preRelease <> gitRev
   where
     gitRev =
       if $(gitHash) == ("UNKNOWN" :: Text) then
@@ -35,10 +36,10 @@ prettyVersion =
 docsVersion :: Text
 docsVersion
   | isPreRelease = "latest"
-  | otherwise    =  "v" <> (T.intercalate "." . map show . take 1 $ versionBranch version)
+  | otherwise    =  "v" <> (T.intercalate "." . map show . take 1 $ version)
 
 
 -- | Versions with two components (e.g., '1.1') are treated as pre-releases.
 isPreRelease :: Bool
 isPreRelease =
-  length (versionBranch version) == 2
+  length version == 2