feat(alias): add warning to avoid unintended duplicate modules

[sapphi-red]

Rollup Plugin Name: alias

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, then include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers:

resolves rollup/rollup#5250

Description

When the alias plugin is configured like this

// rollup.config.js
alias({ entries: { $: 'src' } })

and there a file like

import { test as test1 } from './sub.js';
import { test as test2 } from '$/sub.js';

console.log(test1);
console.log(test2);

https://stackblitz.com/edit/rollup-repro-s5tmli?file=src%2Fmain.js

then, import './sub.js' and import '$/sub.js' resolves to different module. (The same file is included in the bundle twice.)
I believe this is not what people expect. Also IMO the fact that "import 'src/sub.js' doesn't work but $/sub.js works" is difficult to understand.

This PR adds a warning to catch these cases.

The warning will be shown for those that is using the alias plugin to duplicate the module, but I guess no one would do that. It'd be easier to create a different plugin in that case.

[shellscape]

this is great thank you