Add version number to symbol table cache filename

Language/Haskell/GhcMod/Find.hs

@@ -59,8 +59,13 @@ newtype SymbolDb = SymbolDb (Map Symbol [ModuleString])
 
 ----------------------------------------------------------------
 
+-- | When introducing incompatible changes to the 'symbolCache' file format
+-- increment this version number.
+symbolCacheVersion = 0
+
+-- | Filename of the symbol table cache file.
 symbolCache :: String
-symbolCache = "ghc-mod.cache"
+symbolCache = "ghc-mod-"++ show symbolCacheVersion ++".cache"
 
 packageCache :: String
 packageCache = "package.cache"
@@ -130,7 +135,6 @@ getSymbolCachePath = do
 --   if the file does not exist or is invalid.
 --   The file name is printed.
 
--- TODO: Before releaseing add a version number to the name of the cache file
 dumpSymbol :: IOish m => GhcModT m String
 dumpSymbol = do
     dir <- getSymbolCachePath