Use file monitoring for preprocessor re-run (#11411)

This replaces the previous preprocessor re-run logic ("if the
preprocessor output exists and is newer than the input, don't re-run the
preprocessor") with a file monitor, which takes both the input file and
the preprocessor itself into account, solving #11411.

To make this possible, each preprocessor is now tagged with a hash that
uniquely identifies the specific preprocessor version.

The process of finding the actual preprocessor (`runPreProcessor`) to
run has been split off from the actual running; this was necessary so
that we can get a unique hash of the preprocessor (including version)
without having to actually run it, while also avoiding duplicate work
and a potential mismatch between the hash and the actual preprocessor.
Hence, we now have `configurePreProcessor`, which returns a pair of the
former `runPreProcessor` action and the preprocessor hash.

Author: tdammers

Cabal/src/Distribution/Simple/HashValue.hs

@@ -3,6 +3,7 @@
 module Distribution.Simple.HashValue
   ( HashValue
   , hashValue
+  , hashEncode
   , rawHashValue
   , truncateHash
   , showHashValue
@@ -12,6 +13,7 @@ module Distribution.Simple.HashValue
 import Distribution.Compat.Prelude
 import Prelude ()
 
+import qualified Data.Binary as Binary
 import qualified Crypto.Hash.SHA256 as SHA256
 import qualified Data.ByteString.Base16 as Base16
 import qualified Data.ByteString.Char8 as BS
@@ -48,6 +50,10 @@ instance Structured HashValue
 hashValue :: LBS.ByteString -> HashValue
 hashValue = HashValue . SHA256.hashlazy
 
+-- | Hash a value's 'Binary' representation.
+hashEncode :: Binary a => a -> HashValue
+hashEncode = hashValue . Binary.encode
+
 -- | Convert a raw hash value, given as a bytestring, into a 'HashValue'. No
 -- well-formedness guarantees are provided; the caller is responsible for
 -- ensuring that the provided hash is valid.