[dev.cmdgo] cmd/go: add the workspace mode

This change adds the outline of the implementation of the workspace mode.
The go command will now locate go.work files, and read them to determine
which modules are in the workspace. It will then put those modules in
the root of the workspace when building the build list. It supports
building, running, testing, and listing in workspaces. There are still
many TODOs for undone work and other changes to fix certain cases. Some
of these undone parts include: replaces and go.work.sum files, as well
as go mod {test,why,verify}, excludes in workspaces, updating work files
to include module names in comments and setting the GOWORK variable.

For #45713

Change-Id: I72716af7a300a2896087fc8a79c04e951d248278
Reviewed-on: https://go-review.googlesource.com/c/go/+/334934


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>