cmd/go: support overlays in the go command #39958
Description
This has been discussed fairly often, but I figured it was worth filing an issue for tracking purposes.
Many tools, gopls in particular, work on unsaved files. As a result, tools like go/packages support overlays, which can replace file contents or represent entirely new files. The overlay logic for go list in go/packages is complex and error-prone, as it effectively needs to recreate the behavior of go list. It is a constant source of gopls bugs, many of which (#39646 for a recent example) manifest in seemingly unrelated ways. Supporting overlays natively in the go command would eliminate all of this complicated logic and this source of errors.