cmd/go: diagnose import cycles better

Before this CL, the import stack was a) not printed and b) overwritten later
in the build, destroying the information about the cycle. This CL fixes both.

I made time depend on os (os already depends on time) and with this CL the error is:

/Users/r/go/src/pkg/fmt/print.go:10:2: import cycle not allowed
package code.google.com/p/XXX/YYY:
        imports fmt
        imports os
        imports time
        imports os

Doesn't give line numbers for the actual imports, as requested in the bug, but
I don't believe that's important.

Fixes issue 4292.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/13100043