Added a check for unused imports

tests/errors/UnknownPackage.q

@@ -0,0 +1,3 @@
+main() {
+	sys.read()
+}

tests/errors/UnusedImport.q

@@ -0,0 +1,3 @@
+import sys
+
+main(){}

tests/errors_test.go

@@ -40,6 +40,8 @@ var errs = []struct {
 	{"UnknownIdentifier.q", &errors.UnknownIdentifier{Name: "x"}},
 	{"UnknownIdentifier2.q", &errors.UnknownIdentifier{Name: "x"}},
 	{"UnknownIdentifier3.q", &errors.UnknownIdentifier{Name: "x"}},
+	{"UnknownPackage.q", &errors.UnknownPackage{Name: "sys"}},
+	{"UnusedImport.q", &errors.UnusedImport{Package: "sys"}},
 	{"UnusedVariable.q", &errors.UnusedVariable{Name: "x"}},
 }