wrong babel cache using filepath as entry point

Hey folks, thanks for creating this library, I found it very useful.

I'm opening this issue because I found an edge case related to how babel-loader cache works.

babel-loader uses a prop called `cachedIdentifier`. This property determinate if it is necessary or not re-run babel-loader process.

https://github.com/babel/babel-loader/blob/b29516266912fbc50120155fee00efb1b0a19f59/src/index.js#L61

The point here is that it reads the content of the `.babelrc`. All is okay.

The edge case is related with this:

```
{
"plugins": [
["transform-define", "./path/to/config/file.js"]
]
}
```

If you use `babel-plugin-transform-define` specifying a file path, and then you modify the content of the file, in the next babel-loader it will be determinate that a new run is not necessary, so it will be served the last result from the cache that it's different from your expected result!

Literally, I was thinking I was crazy until discovering this.

The fix could be easiest as determinate the new way to calculate `cachedIdentifier`. The problem is, because in my case I'm under Next.js, that's is not something easy to do because Next.js internal doesn't contemplate this modification.

Ideally, we should do that transparently adding a bit of code inside `babel-plugin-transform-define` to detect the edge case, but not sure if we can do that, need to investigate around babel-loader API inside a plugin.

My current workaround is remove the cache before init the process:

```
rm -rf node_modules/.cache/babel-loader && npm run dev
```
but obviously, this is a very bad solution and slow.

any idea? 🙂