Following discussion on golang.org/issue/26334, this CL changes
the GOPROXY environment setting to be a list of proxies, tried in
sequence. The first successful or non-404/410 error is taken as
authoritative. Otherwise the next proxy is tried, and so on.
As in earlier releases, GOPROXY=direct means "connect directly",
but now it can appear in a longer list as well.
This will let companies run a proxy holding only their private modules
and let users set GOPROXY=thatproxy,publicproxy or GOPROXY=thatproxy,direct
to fall back to an alternate mechanism for fetching public modules.