Reset the cache-dir when a component is added to HscEnv

[fendor]

Also, adds a prefix for the cache-dir: InstalledUnitId.

closes #15
closes #13

[mpickering]

This patch looks as I imagined it. We really need to refactor that 6-tuple into a record now!

[mpickering]

We discussed in IRC about what needs to happen for this patch to be finished.

• A comment about the invariants which need to be preserved about the cache directories and about why multiple mains are not an issue.
• Try to use just the filtered out inplace packages from the options in the salt rather than all components currently loaded.

[fendor]

Indeed and I am on it.

[mpickering]

pretty sure it does.
In my logs:

"Making new HscEnv[\"main\"]"

Which is from:

            print ("Making new HscEnv" ++ (show inplace))

Right it does, but it might as well not as removeInplacePackage will never match on main as no components will ever depend on main. My idea instead was to look at which components removeInplacePackage actually removed and use those as the salt.

[fendor]

Ready to merge, in my opinion.
cc @mpickering

[mpickering]

So the conclusion is it fixes some issues but there are still problems?

[fendor]

for this pr, yes, but I will continue to venture into the problem on Cabal.
For future reference this error https://gist.github.com/fendor/93d8d7685a1c09371e6e2f1ed09bd26b
can be produced by opening cabal-install, followed by Cabal and then reopening only cabal-install component.

[mpickering]

Would be good if we could reproduce it on some smaller projects other than GHC and Cabal..

[fendor]

I will provide a minimal reproducible project. I have a good idea how to reproduce it.

[fendor]

A pretty small example: https://github.com/fendor/ghcide-bad-interface-files