Fix quadratic memory usage in GetLocatedImports

[mpickering]

At startup GetLocatedImports is called on all known files. Say you have 10000 modules in your project then this leads to 10000 calls to GetLocatedImports running concurrently.

In GetLocatedImports the known targets are consulted and the targetsMap is created by mapping the known targets. This map is used for introducing sharing amongst filepaths. This operation copies a local copy of the target map which is local to the rule.

let targetsMap = HMap.mapWithKey const targets

So now each rule has a hashmap of size 10000 held locally to it and depending on how the threads are scheduled there will be 10000^2 elements in total allocated in hashmaps. This used a lot of memory.

Solution: Return the normalising map in the result of the GetKnownTargets rule so it is shared across threads.

Fixes #4317

[michaelpj]

Please write down this subtle reasoning in the code! Otherwise we will be unlikely to retain this property over time.

[fendor]

I added the comment to the documentation of 'normalisingMap'.