Avoid parsing sources for dependencies

[pepeiborra]

Now that ghcide is using interface files, it would be desirable if ghcide parsed only the module headers to spy out the imports and skipped the rest of the file, just like GHC does.

[pepeiborra]

I have started working on this, continuing with a previous effort at pepeiborra/ghcide#1

[mpickering]

Does parsing the imports require an HscEnv? It would be good for multi-component if it didn't.

[pepeiborra]

It requires some DynFlags, but I think not an HscEnv

[pepeiborra]

Scratch that, an HscEnv is required to preprocess the source files before parsing the imports.

[pepeiborra]

I have a working implementation now, planning to send a PR soon. All the uses of GetParsedModule removed; haddocks are now embedded in .hi files, and ghcide doesn't even need to watch workspace files anymore.

Some early measurements suggest an important startup time improvement:

• before: 1m20s
• after: < 30s

Memory usage does not seem to improve that much

[mpickering]

Nice one @pepeiborra If you point me to your branch I will do a review.

[pepeiborra]

Branch is https://github.com/pepeiborra/ghcide/tree/mod-summary but it's not ready for review yet. I plan to clean up and submit a PR today or tomorrow