cmd/go/internal/modload: spurious "package provided … at latest version but not at required version" errors

[bcmills]

There appears to be a bug in our logic for determining whether a package can be resolved by adding a module.

This loop sets newMissingVersion on the returned ImportMissingError if any of the candidates returned by QueryPackage is required at a higher version in the build list.

However, if there are multiple (nested) candidate modules and only one is required at a higher version (as in the case of a module split without a requirement cycle; see also #27899), we should suggest adding one of the candidates that is viable, rather than rejecting all of them with a newMissingVersion error.

This bug was found by code inspection while working on #36460, so I still need to find a test case that reproduces it.

CC @matloob @jayconrod

[gopherbot]

Change https://golang.org/cl/251446 mentions this issue: cmd/go/internal/modload: fix spurious import resolution error

[bcmills]

Fix pending, with a regression test that successfully reproduced the bug as expected.