The completion function belongs in internal/lsp/source, so move it
there. Some small refactoring of completion, by moving each type of
completion into helper functions that append to the list of results.
Change-Id: I8599092906609591d499183657fe2d21d1f74df1
Reviewed-on: https://go-review.googlesource.com/c/148397
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
