Mention 'Path_* module in package.yaml`

Problem: after we started using mixins feature, generated .cabal file
requires cabal 2.0. Attempt to upload our package to Hackage now reports
an error, saying that `autogen-modules` section have to mention `Path_*`
files.

Solution: add `Paths_xrefcheck` to `generated-other-modules` section of
`package.yaml`.

package.yaml

@@ -94,10 +94,15 @@ dependencies:
 library:
   source-dirs: src
 
+  generated-other-modules:
+  - Paths_xrefcheck
+
 executables:
   xrefcheck:
     main:                Main.hs
     source-dirs:         exec
+    generated-other-modules:
+    - Paths_xrefcheck
     ghc-options:
     - -threaded
     - -rtsopts
@@ -110,6 +115,8 @@ tests:
   xrefcheck-tests:
     main:                Main.hs
     source-dirs:         tests
+    generated-other-modules:
+    - Paths_xrefcheck
     dependencies:
     - xrefcheck
     - hspec