x/tools/gopls: improve command-line UX when checking files outside of the current module

[pjweinb]

$ go version
go version go1.15.6 darwin/amd64

To reproduce, in the tools repository:

$ cd gopls
$ gopls check ../internal/lsp/cmd/cmd.go
2020/12/29 08:28:38 Error:2020/12/29 08:28:38 warning: diagnose package: no package for golang.org/x/tools/internal/lsp/cmd
	snapshot=1
	package="golang.org/x/tools/internal/lsp/cmd"

but it never finishes. Contrast this with

$ cd gopls
gopls check main.go

which returns immediately.