builder: fix a bug with submodule import (#10318)

examples/submodule/main.v

@@ -0,0 +1,7 @@
+import mymodules { add_xy }
+import mymodules.submodule { sub_xy }
+
+fn main() {
+	println(add_xy(2, 3)) // expected: 5
+	println(sub_xy(10, 7)) // expected: 3
+}

examples/submodule/mymodules/main_functions.v

@@ -0,0 +1,5 @@
+module mymodule
+
+pub fn add_xy(x int, y int) int {
+	return x + y
+}

examples/submodule/mymodules/submodule/sub_functions.v

@@ -0,0 +1,5 @@
+module submodule
+
+pub fn sub_xy(x int, y int) int {
+	return x - y
+}

vlib/v/util/module.v

@@ -32,7 +32,7 @@ pub fn qualify_module(pref &pref.Preferences, mod string, file_path string) stri
 	if mod == 'main' {
 		return mod
 	}
-	clean_file_path := file_path.all_before_last('/')
+	clean_file_path := file_path.all_before_last(os.path_separator)
 	// relative module (relative to working directory)
 	// TODO: find most stable solution & test with -usecache
 	if clean_file_path.replace(os.getwd() + os.path_separator, '') == mod {
@@ -110,5 +110,13 @@ pub fn mod_path_to_full_name(pref &pref.Preferences, mod string, path string) ?s
 			}
 		}
 	}
+	if os.is_abs_path(pref.path) && os.is_abs_path(path) && os.is_dir(path) { // && path.contains(mod )
+		rel_mod_path := path.replace(pref.path.all_before_last(os.path_separator) +
+			os.path_separator, '')
+		if rel_mod_path != path {
+			full_mod_name := rel_mod_path.replace(os.path_separator, '.')
+			return full_mod_name
+		}
+	}
 	return error('module not found')
 }