internal/lsp: modify check for a missing package
internal/lsp: modify check for a missing package

This change moves from marking a package with `go list` errors as
missing, to marking a package with no files as missing.

Change-Id: Ibad1e67518d8a7f4c4bde416c53ab8132ae534e3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/184039
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools