Remove internal library to work around stack bug

Using an internal library led to an error with "stack ghci" that also affected by IDE. It seems to be related to this stack issue, but wasn't fixed by running "stack build" first:

commercialhaskell/stack#4148

prolog.cabal

@@ -15,7 +15,7 @@ common prolog-common
         containers,
         base
 
-library prolog-lib
+library
     import: prolog-common
     hs-source-dirs: src
     exposed-modules:
@@ -27,12 +27,12 @@ library prolog-lib
 executable prolog
     import: prolog-common
     main-is: src/Main.hs
-    build-depends: prolog-lib
+    build-depends: prolog
 
 test-suite prolog-tests
     import: prolog-common
     type: exitcode-stdio-1.0
     main-is: src/Prolog/Test.hs
     build-depends:
-        prolog-lib,
+        prolog,
         HUnit