internal/typeparams: hide go1.18 API usage behind a build constraint

Temporarily hide usage of the new go/types API, so that we can safely
tag x/[email protected] without risk of further breakage when these APIs
change.

Updates golang/go#48632

Change-Id: Idba02d09644622b3d973a684a76514c86eefa17f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/352854
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>

gopls/api-diff: create api-diff command for gopls api

This change adds a command that can be used to see the difference in
API between two gopls versions. It prints out the changes in a way that
can be copy-pasted into the release notes.

Also, only run the copyright test with 1.18. I wanted to do this before
to use filepath.WalkDir, but now it also doesn't work with generic
syntax (it doesn't use packages.Load, so doesn't respect build tags).

Fixes golang/go#46652

Change-Id: I3670e0289a8eeaca02f4dcd8f88f206796ed2462
Reviewed-on: https://go-review.googlesource.com/c/tools/+/327276
Trust: Rebecca Stambler <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>