gopls/internal/golang: SemanticTokens: edits for clarity

This CL applies a number of readability enhancements:

- eliminate unnecessary local variables
- reorder and document struct fields
- strength-reduce cache.Package to metadata.Package
- more conventional variable names. e.g.
   items -> tokens
     lng -> length
      ti -> info
       x -> id
       y -> obj, ancestor
      nd -> parent
 def/use -> obj
- rename unexpected() to errorf(), with printf semantics
- remove unreachable cases
  (e.g. info == nil; impossible types.Object subtypes)
- use is[T] to eliminate statements
- eta-reduce inspector function
- add doc comments
- add TODOs for further simplification and suspected bugs.

This is a pure refactoring; no behavior changes are intended.

Change-Id: I97a8f0b76008b0f784328f155f41bea8799f8389
Reviewed-on: https://go-review.googlesource.com/c/tools/+/561056
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan