Ignore non-existent directories running ghc-pkg

When creating a merged package database of dependencies (only done on Windows), packages
are checked for validity. This includes empty packages such as bytestring-builder which
would fail:
```
bytestring-builder-0.10.8.2.0: import-dirs: C:\_bzl\ptbwskyc\execroot\rules_haskell_examples\bazel-out\x64_windows-fastbuild\bin\external\stackage/externalZSstackageZSbytestring-builder/_iface doesn't exist or isn't a directory (use --force to override)
```

Author: avdv

haskell/private/cabal_wrapper.py

@@ -245,7 +245,7 @@ def distdir_prefix():
         for db in deps_package_databases:
             try:
                 ps = subprocess.Popen([ghc_pkg, "dump", "--expand-pkgroot", "-f", os.path.dirname(os.path.join(execroot,db))], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-                subprocess.check_output([ghc_pkg, "update", "-f", deps_package_db, "-"], stdin=ps.stdout, stderr=subprocess.PIPE)
+                subprocess.check_output([ghc_pkg, "update", "--force", "-f", deps_package_db, "-"], stdin=ps.stdout, stderr=subprocess.PIPE)
                 ps.wait()
             except subprocess.CalledProcessError as err:
                 sys.stdout.buffer.write(err.stdout)