Compatibility with pnpm

Hi everyone,

I'm a big fan of pnpm, and I wanted to use it with a project using karma as test runner and I noticed that karma was not compatible with pnpm.

I debugged my config and I found that:

- When karma is loading plugins:
- It try to guess the plugin directory. It works by using the `lib/plugin.js` directory and go two directories up to retrieve the `node_modules` directory ([here](https://github.com/karma-runner/karma/blob/50f963551fbccf669c8d42499d0406ded2ec6176/lib/plugin.js#L34)).
- It works with npm (and probably with yarn too) because karma is usually installed inside the project node_modules directory.
- Unfortunately, with pnpm, it does not work like this:
- Karma is not directly installed in the project `node_modules`, it is installed in a `.pnpm` directory inside the project `node_modules` directory. The directory tree will look like this:

```
[project]
node_modules
.pnpm
karma@6.4.6
node_modules
karma
lib
plugin.js
karma-chrome-launcher@3.1.1
node_modules
karma-chrome-launcher
karma -> .pnpm/karma@6.4.1/node_modules/karma
karma-chrome-launcher -> .pnpm/karma-chrome-launcher@3.1.1/node_modules/karma-chrome-launcher
```

➡️ By going two directories up, the plugin directory is then `[project]/.pnpm/karma@6.4.1/node_modules`, so obviously no plugins will be detected.

A (probably very naive) fix would to use `process.cwd()` instead of `__dirname` of `karma/lib/plugin.js`, this way you would get the project root directory. But, as said, it's probably very naive, and I'm sure I probably miss a lot of use cases.

In any case, I'd be happy to submit PR to fix this issue if you think it's worth it.

Start by reading lib/plugin.js and reproduce plugin loading with the pnpm node_modules layout described in the issue. Verify how the plugin directory is determined and make sure the finished change detects plugins with pnpm without breaking the existing npm-style layout.

Written by the indexing model from the issue text.