[Improve] Use meta path instead of AST transform to implement new config.

[mzr1996]

Motivation

In the original new config implementation, we use AST transform to manipulate the import statements.
It's a little obscure, and hard to debug.

Modification

Python provide a native feature to override the import behaviors. In this PR, we use sys.meta_path to
implement the lazy import and base config file import.

BC-breaking

I have tested on MMPreTrain new config, it shouldn't change the original usages, but please check it carefully.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.