fix: dts redirect extension should add correctly

[Timeless0911]

Summary

This PR fix some cases about redirect.dts.extension:

1. When path is not redirected to a absolute/relative path (maybe redirect.dts.path: false is set), we should not add extension.
2. When path is redirected to ./, we should remove the / prefix first
3. When path is redirected to entry of itself via paths in tsconfig.json, we should get correct value of isOutsideRootdir to make the path redirect to correct relative path from root.

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).