tools: fix windows install of an already existing module with vpm (#19761)

Author: ttytm

cmd/tools/vpm/install.v

@@ -184,10 +184,18 @@ fn vpm_install_from_vcs(modules []string, vcs &VCS) {
 				if os.exists(minfo.final_module_path) {
 					eprintln('Warning module "${minfo.final_module_path}" already exists!')
 					eprintln('Removing module "${minfo.final_module_path}" ...')
-					os.rmdir_all(minfo.final_module_path) or {
+					mut err_msg := ''
+					$if windows {
+						os.execute_opt('rd /s /q ${minfo.final_module_path}') or {
+							err_msg = err.msg()
+						}
+					} $else {
+						os.rmdir_all(minfo.final_module_path) or { err_msg = err.msg() }
+					}
+					if err_msg != '' {
 						errors++
 						eprintln('Errors while removing "${minfo.final_module_path}" :')
-						eprintln(err)
+						eprintln(err_msg)
 						continue
 					}
 				}

cmd/tools/vpm/install_test.v

@@ -52,11 +52,6 @@ fn test_install_from_git_url() {
 }
 
 fn test_install_already_existent() {
-	// FIXME: Skip this for now on Windows, as `rmdir_all` results in permission
-	// errors when vpm tries to remove existing modules.
-	$if windows {
-		return
-	}
 	mut res := os.execute('${v} install https://github.com/vlang/markdown')
 	assert res.exit_code == 0, res.output
 	assert res.output.contains('already exists')