cmd/go: add support for vendoring in workspace mode
In most cases this change removes assumptions that there is a single
main module in vendor mode and iterates over the workspace modules
when doing checks. The go mod vendor command will now, if in workspace
mode, create a vendor directory in the same directory as the go.work
file, containing the packages (and modules in modules.txt) loaded from
the workspace. When reassembling the module graph from the vendor
directory, an edges are added from each of the main modules to their
requirements, plus additionally to a fake 'vendor/modules.txt' module
with edges to all the modules listed in vendor/modules.txt.