contrib: Add Go multimod workspace setup script.

[davecgh]

This adds a script to the contrib directory that initializes a Go multi-module workspace and adds all of the modules provided by the dcrd repository to it on an as needed basis. Note that workspaces require Go 1.18+.

This is useful when developing across multiple modules in the repository and allows development environments that make use of the Go language server (aka gopls), such as VSCode, to provide full support without also needing to temporarily create replacements in the various go.mod files or individually add every module.

Do note, however, that workspaces are local, so final submissions to the repository will still require the appropriate changes to the relevant go.mod files to ensure resolution outside of the workspace.

Finally, it also updates the README.md in the contrib directory accordingly.

[chappjc]

Looks good. One question - will replace statements in go.mod be a thing of the past once we drop support for go 1.17?

[chappjc]

Would be be desirable to also add the cmd app modules to the workspace?

[davecgh]

Looks good. One question - will replace statements in go.mod be a thing of the past once we drop support for go 1.17?

Perhaps in the main go.mod (and then CI would need to create the workspace to ensure the latest code is tested). However, any modules in development that rely on other unreleased modules will necessarily still need them if committed to the repository. That is mainly what I was referring to with the "Do note, however, that workspaces are local, so final submissions to the repository will still require the appropriate changes to the relevant go.mod files to ensure resolution outside of the workspace."

Would be be desirable to also add the cmd app modules to the workspace?

I'm not sure what you mean here. The apps under cmd aren't separate modules.

[chappjc]

The apps under cmd aren't separate modules

Ah, whoops, I have a couple in dcrdata/cmd as their own modules. My mistake.

[chappjc]

Looks good. The script works. I've switched to go workspaces and gopls recently and it's great.

---------------------- contrib/dcr_setup_go_workspace.sh ----------------------
new file mode 100755
index 00000000..82131267