Expected Behaviour

When creating and disposing multiple webpack instances using ts-loader, compiler instances should not be retained.

Actual Behaviour

When running a build with ts-loader, webpack compiler instances are never removed by the GC.

Steps to Reproduce the Problem

1. create a minimal ts-loader configuration like the one from the readme
2. create a webpack compiler and call run() on it without storing any references
3. repeat step 2 enough times and the node process will run out of memory

Location of a Minimal Repository that Demonstrates the Issue.

https://github.com/valerio/ts-loader-leak

Additional details

I have an application that runs webpack in-memory to transpile some TypeScript code.
The webpack compilers are disposed after some time, but I noticed that references to them were still being held by ts-loader.

I've spent some time debugging this and I saw that these references are held by the webpackInstances array:

The array is declared here: https://github.com/TypeStrong/ts-loader/blob/master/src/index.ts#L30
This is a global variable and references are only ever added to it, never removed, as far as I can see.

The array is used to determine a key for an instance cache, which is using a WeakMap: https://github.com/TypeStrong/ts-loader/blob/master/src/index.ts#L189-L196

I think this array could be replaced with another WeakMap itself (or use WeakRef) so it doesn't retain compiler instances.