checker: verify that there is a main module

vlib/v/checker/checker.v

@@ -89,7 +89,9 @@ pub fn (mut c Checker) check_files(ast_files []ast.File) {
 		// shared libs do not need to have a main
 		return
 	}
-	if has_main_mod_file && !has_main_fn {
+	if !has_main_mod_file {
+		c.error('projet must include a `main` module or be a shared library (compile with `v -shared`)', token.Position{})
+	} else if !has_main_fn {
 		c.error('function `main` must be declared in the main module', token.Position{})
 	}
 }

vlib/v/checker/tests/no_main_mod.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/no_main_mod.v:1:1: error: projet must include a `main` module or be a shared library (compile with `v -shared`)
+    1 | module a
+      | ^

vlib/v/checker/tests/no_main_mod.vv

@@ -0,0 +1 @@
+module a