x/tools/gopls: investigate better support for CGO_ENABLED=0

[findleyr]

This is a speculative issue: we may be able to improve support for working with CGO_ENABLED=0 (or GOOS=..., as is set automatically by the solution to #29202).

For example, we could investigate using https://pkg.go.dev/go/types#Config.FakeImportC. I don't yet know if this is feasible, but it could make gopls more usable in scenarios where a c toolchain is unavailable, or prohibitively slow.

[gopherbot]

Change https://go.dev/cl/592578 mentions this issue: gopls/internal/cache: get FakeImportC working for CGO workspaces