cmd/go: "go list -f '{{.Stale}}'" stack overflow with cyclic imports [1.15 backport]

[gopherbot]

@stamblerre requested issue #25830 to be considered for backport to the next 1.15 minor release.

@gopherbot please consider this for backport to 1.16 and 1.15

[gopherbot]

Change https://golang.org/cl/336669 mentions this issue: [release-branch.go1.15] cmd/go/internal/load: always set IsImportCycle when in a cycle

[bcmills]

According to @stamblerre we're still getting a nonzero rate of reports for this crash among gopls users.

The fix CL is small (and low-risk) and the TryBots are happy with it (and the corresponding new test), so this seems worth backporting.

[gopherbot]

Closed by merging b81d75f to release-branch.go1.15.