Parse sources of conditional env modules.

Do nothing about it yet, but trigger an error if env/default.ak is
  missing; but only if there's any module at all under env.

crates/aiken-lang/src/ast.rs

@@ -28,6 +28,7 @@ pub type UntypedModule = Module<(), UntypedDefinition>;
 pub enum ModuleKind {
     Lib,
     Validator,
+    Env,
 }
 
 impl ModuleKind {
@@ -38,6 +39,10 @@ impl ModuleKind {
     pub fn is_lib(&self) -> bool {
         matches!(self, ModuleKind::Lib)
     }
+
+    pub fn is_env(&self) -> bool {
+        matches!(self, ModuleKind::Env)
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]

crates/aiken-lang/src/tipo/environment.rs

@@ -785,7 +785,9 @@ impl<'a> Environment<'a> {
                         })?;
 
                 if module_info.kind.is_validator()
-                    && (self.current_kind.is_lib() || !self.current_module.starts_with("tests"))
+                    && (self.current_kind.is_lib()
+                        || self.current_kind.is_env()
+                        || !self.current_module.starts_with("tests"))
                 {
                     return Err(Error::ValidatorImported {
                         location: *location,