rego: Return true for file.exists for directories too (#2191)

I'm not sure why we made it to return `true` only for non-directory
files. But I think it makes sense to have it be `true` for directories
as well.

internal/engine/eval/rego/lib.go

@@ -77,18 +77,14 @@ func FileExists(res *engif.Result) func(*rego.Rego) {
 			fs := res.Fs
 
 			cpath := filepath.Clean(path)
-			finfo, err := fs.Stat(cpath)
+			_, err := fs.Stat(cpath)
 			if err != nil {
 				if errors.Is(err, os.ErrNotExist) {
 					return ast.BooleanTerm(false), nil
 				}
 				return nil, err
 			}
 
-			if finfo.IsDir() {
-				return ast.BooleanTerm(false), nil
-			}
-
 			return ast.BooleanTerm(true), nil
 		},
 	)