QL: add query detecting use of db-types outside the lib folder

ql/ql/src/queries/style/DBTypeInNonLib.ql

@@ -0,0 +1,24 @@
+/**
+ * @name Use of database type outside the language core
+ * @description Database types should only be used in the language core, abstractions should be used elsewhere.
+ * @kind problem
+ * @problem.severity warning
+ * @id ql/db-type-outside-core
+ * @tags maintainability
+ * @precision very-high
+ */
+
+import ql
+
+from TypeExpr te
+where
+  te.isDBType() and
+  not te.getLocation().getFile().getAbsolutePath().matches("%/lib/%") and
+  exists(File f | f.getAbsolutePath().matches("%/lib/%")) and
+  // it is needed in one case.
+  not te = any(Class c | c.getName() = "SuppressionScope").getASuperType() and
+  // QL-for-QL only has a src/ folder.
+  not te.getLocation().getFile().getAbsolutePath().matches("%/ql/ql/%") and
+  // tests are allowed to use DB types.
+  not te.getLocation().getFile().getAbsolutePath().matches("%/test/%")
+select te, "Database type used outside the language lib/ folder."