Move workspace symbol implementation out of SourceKitLSPServer.swift

The `workspace/symbol`, `workspace/symbolNames`, `workspace/symbolInfo`
and `workspaceSymbol/resolve` implementations made up roughly 470 lines
of SourceKitLSPServer.swift. Move them to WorkspaceSymbols.swift and
leave each request handler in SourceKitLSPServer.swift as a single
statement that forwards to its implementation.

Also remove `languageClass(for:)`, whose last caller went away when
language service instances became scoped per workspace.

Author: rintaro

Sources/SourceKitLSP/CMakeLists.txt

@@ -32,6 +32,7 @@ add_library(SourceKitLSP STATIC
   TestDiscovery.swift
   TextEdit+IsNoop.swift
   Workspace.swift
+  WorkspaceSymbols.swift
 )
 set_target_properties(SourceKitLSP PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY}