x/tools/gopls: mod tidiness is a precondition for diagnostics

[adonovan]

If you build gopls at today's head and check out a new github.com/kubernetes/kubernetes repo, then run the 'check' subcommand, it fails because of the go.mod tidiness check:

kubernetes$ gopls check ./cmd/kubectl/kubectl.go 
2022/10/24 11:19:00 Error:2022/10/24 11:19:00 tidy: diagnosing file:///Users/adonovan/w/kubernetes/go.mod: err: exit status 1: stderr: go: bitbucket.org/bertimus9/systemstat@v0.0.0-20180207000608-0eeff89b0690: module lookup disabled by GOPROXY=off
go: downloading github.com/onsi/ginkgo v1.14.0
...many more...
go: downloading golang.org/x/tools v0.1.10-0.20220218145154-897bd77cd717
go: k8s.io/code-generator@v0.0.0 requires
	github.com/go-openapi/jsonreference@v0.19.5: module lookup disabled by GOPROXY=off

2022/10/24 11:19:00 Error:2022/10/24 11:19:00 tidy: diagnosing file:///Users/adonovan/w/kubernetes/go.mod: err: exit status 1: stderr: go: bitbucket.org/bertimus9/systemstat@v0.0.0-20180207000608-0eeff89b0690: module lookup disabled by GOPROXY=off
go: downloading github.com/armon/go-socks5 v0.0.0-20160902184237-e75332964ef5
...
go: k8s.io/code-generator@v0.0.0 requires
	github.com/go-openapi/jsonreference@v0.19.5: module lookup disabled by GOPROXY=off

This remains true even after a successful go build ./cmd/kubectl/ to download modules. Running go mod tidy fixes the problem.

gopls should not make mod tidiness a precondition for analysis.