proposal: cmd/go: add go workspace exclude directories

[switchupcb]

Step 1: https://github.com/golang/proposal#the-proposal-process
The proposal author creates a brief issue describing the proposal.
Note: There is no need for a design document at this point.
Note: A non-proposal issue can be turned into a proposal by simply adding the proposal label.
Note: Language changes should follow a separate template

First, I want to shout out @matloob for being a baller and solving #56697. He singlehandedly saved Google engineers millions of hours in error debugging time cause of this change. THANKS MATLOOB!

Context

https://go.dev/ref/mod#workspaces

Go workspace is used to ensure a module in the workspace uses the import of the other module in the workspace.

Usecase

In Disgo, I want to be able to exclude ./wrapper and _gen from Go Workspace defined:

go 1.18

use (
	./
	./_examples
	./cache
	./shard
)

Can't do it or am missing something cause of the ./ which is necessary for the main package.

Exclude Directories

Most build tools allow the exclusion of directories. Even GNU Make that was made in 1977. I'm not sure what else to say to start this proposal. If there is a reason to not allow the exclusion of files, it's not clear. Perhaps, I am doing something incorrect. I'm just lost...

[seankhliao]

_gen is already its own module, without being listed in go.work, it is not part of the workspace.
wrapper is a package of the root module, it doesn't make sense to break up a module through a workspace.

In general, the exclusions are the same as for modules: carve out a nested module if necessary.

As there are no implicit includes in workspaces, it doesn't make sense to need an exclude directive.

[switchupcb]

@seankhliao Why do I have to specify GOWORK=off in #56697 (comment) for that folder?