ScalaMtags computes a wrong symbol for members in package objects

[gabro]

Given

package object K {
  val x = 42
}

the symbol for x should be _root.K.package.x. Right now it's _root_.K.x

[olafurpg]

The expected symbol should be

_root_.scala.`package`.Either#

which is a type alias. However, I think we index that symbol as _root_.scala.Either in scalamtags. We need to add package to the owner when entering package objects in https://github.com/scalameta/language-server/blob/1299a44633f5f36f757ad26a08a52f53bc712b6c/metaserver/src/main/scala/scala/meta/languageserver/mtags/ScalaMtags.scala#L26

[olafurpg]

A better way to test this is to update this line here
https://github.com/scalameta/language-server/blob/1299a44633f5f36f757ad26a08a52f53bc712b6c/metaserver/src/test/scala/tests/mtags/ScalaMtagsTest.scala#L42
to expect _root_.a.b.c.K.package.l.

[olafurpg]

I rebased this PR on top of #107 and fixed the bug

[gabro]

Nice! Let's rebase this on master once #107 is in and then merge

[olafurpg]

PSA it's necessary to clean the index cache to pickup this change as described here https://github.com/scalameta/language-server/blob/master/CONTRIBUTING.md#troubleshooting

Once we move to sql we can probably version the index so algorithmic changes to mtags bump up the version and invalidate older caches.