FIX: Modules were attempting to conver std imports into absolute paths.

integration_tests/modules_test.ucg

@@ -2,6 +2,11 @@ let test_empty_mod = module {
 } => {
 };
 
+let std_lib_import = module {
+} => {
+    let tpl = import "std/tuples.ucg";
+};
+
 let empty_mod_instance = test_empty_mod{};
 
 let test_simple_mod = module {

src/ast/mod.rs

@@ -545,6 +545,10 @@ impl ModuleDef {
             }
             if let Expression::Import(ref mut def) = e {
                 let path = PathBuf::from(&def.path.fragment);
+                // std/ paths are special and do not get made into absolute paths.
+                if path.starts_with("std/") {
+                    return;
+                }
                 if path.is_relative() {
                     def.path.fragment = base
                         .join(path)