Upgrade to Scalameta v4.0

[olafurpg]

Currently, Metals is on the latest 2.x release of Scalameta which is from January this year. Since then there have been a ton of improvements in the semanticdb-scalac compiler plugin including

• more lightweight default settings, it no longer persists info on referenced symbols
• plenty of bug fixes

Upgrading will require a decent refactoring of the codebase, which I think we should go ahead with before cutting a v0.1 release.

• Complete Scalameta v4.0 milestone https://github.com/scalameta/scalameta/milestone/26
• Publish Scalafix v0.6
• Pick up where Upgrade to Scalameta v3.7 #275 left.

[olafurpg]

I am working on getting a scalafix release out this week.

[olafurpg]

Currently blocked by scalacenter/scalafix#675

We should be able to go ahead with the upgrade ignoring scalafix related problems.

[olafurpg]

(hopefully) Last blocking PR for upgrade scalameta/scalameta#1478 fixing a cross-platform bug that was caught on appveyor. We should really setup a CI on windows

[olafurpg]

I'll try to take a stab at this tomorrow, had good success today migrating metadoc scalameta/metabrowse#90

[olafurpg]

The last progress in this is the PR #275. The migration on the Metals side is mostly complete but it the upgrade didn't get merged because it was a regression compared to current master. For example, hover broke after incremental compilation, definition failed for static Java inner classes macro annotation definitions

These issues are being addressed in Scalameta v4.0, see the v4.0 milestone to track progress https://github.com/scalameta/scalameta/milestone/26

[olafurpg]

Fixed in #332