cmd/go: get panics with "can't find reason for requirement on" [1.17 backport]

[gopherbot]

@bcmills requested issue #47979 to be considered for backport to the next 1.17 minor release.

@gopherbot, please backport to Go 1.17. This is a cmd/go panic due to an internal invariant violation, which may be confusing for end users to diagnose.

[bcmills]

Marking as release-blocker: I have a fix and expect to merge it soon, so it would be a bit of a shame for this fix to miss the boat.

[gopherbot]

Change https://golang.org/cl/348411 mentions this issue: [release-branch.go1.17] cmd/go/internal/modload: scan dependencies of root paths when raising version limits in editRequirements

[gopherbot]

Closed by merging 328cf2e to release-branch.go1.17.