Breaks the internal GTD infrastructure into seperate symbols for
determining the navigable symbol at the caret position and performing
the navigation.

Clear completion pattern matcher caches on solution close

Fixes
https://devdiv.visualstudio.com/DevDiv/_workitems?id=391624&_a=edit

- Inline Rename fields now consist of a themeable/customizable
background, foreground, and border. The border is used in High Contrast.
- Resolved and Unresolved conflict borders can now be themed &
customized.
- Resolved and Unresolved conflict borders now have different dash
styles so they are distinct from each other and regular Inline Rename
fields. The Dashboard shows a key indicating which type of conflict
corresponds to which border dash style.

Fixes #18792

Also updates other references based on Preview 1 and moves from
Microsoft.Composition to System.Composition

See dotnet/roslyn#19368

This reverts commit 8262112e, reversing
changes made to 837ca214.

This is an automated migration of our project files to the new SDK /
PackageReference.  It is done using the tool located here:

https://github.com/jaredpar/ConvertPackageRef

(and there's a destructor present)

Fixes https://github.com/dotnet/roslyn/issues/16567