go/internal/gcimporter: error with embedded non-interface (possibly an incomplete type)

[griesemer]

To reproduce:

1. go get github.com/google/go-cmp/cmp
2. cd $GOPATH/src/github.com/google/go-cmp/cmp
3. go test

# github.com/google/go-cmp/cmp_test
./compare_test.go:23:2: could not import github.com/google/go-cmp/cmp (cannot import, possibly version skew (embedded type is not an interface) - reinstall package)
./compare_test.go:481:23: cannot range over p (invalid operand)
vet: typecheck failures
FAIL	github.com/google/go-cmp/cmp [build failed]

[griesemer]

Appears to be related to fix for #25301.

[griesemer]

This is probably a duplicate of #25596.

[gopherbot]

Change https://golang.org/cl/115155 mentions this issue: go/types: don't over-eagerly verify embedded interfaces

[myitcv]

Thanks very much @griesemer!