x/tools/gopls: set types.Config.GoVersion based on go.mod

[findleyr]

In Go 1.18, the GoVersion was added to go/types.Config (proposal: #46648). This field configures the Go language version, as would be passed to the compiler via -lang, and read by the Go tool from the go directive in the go.mod.

This means that gopls can now match compiler type-checking errors exactly, by setting Config.GoVersion based on the applicable go.mod file. We should do this for 1.18, as it will be common for users to write generic code with gopls, only to be surprised when they get errors from the go command due to their go.mod file.

This is also a critical component of using gopls built with a recent version of Go to target building at an older Go version.

[findleyr]

As a bonus, it would be cool to offer a quick-fix that updates the go.mod file to the required version. That is a much lower priority.

[findleyr]

CC @hyangah @heschi

Hana was (quite wisely) concerned about users bumping up against version skew pain with 1.18. This is one part fixing that.

[gopherbot]

Change https://golang.org/cl/383394 mentions this issue: internal/lsp/cache: set types.Config.GoVersion