x/tools/gopls: don't access the network for `go mod tidy` diagnostics

[stamblerre]

A few reports of issues with gopls when the user has no internet (one on Slack, one on microsoft/vscode-go#3105).

I wonder if there is a way we can have a regression test this - maybe by configuring something in the environment? @findleyr

[findleyr]

Following https://golang.org/cl/228235, the regtests use a proxy within our control. If we have examples of how gopls breaks without a network connection, we should be able to reproduce.

[mingoal]

@findleyr
suggest scenario:
in go file, include one package which has no local cache,

1. without network connection, will gopls hang or not?
2. with network connection which requires http proxy, will gopls hang or not?

[gopherbot]

Change https://golang.org/cl/274120 mentions this issue: internal/lsp: disable network access for some go commands