design: update gopls workspace design with implementation details
design: update gopls workspace design with implementation details

Adding a few more concrete details after discussion today with
Heschi.

Change-Id: I0216eaa584d8a9048c5138926b2bfad8926cc034
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/248405
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/proposal,golang/proposal