If we're looking for a module for a/b/c/d/e,
we check for a module named a/b/c/d/e,
then a/b/c/d, then a/b/c, then a/b, then a.
If we know the source repo for a/b/c and that
fails, we should report that error instead of
continuing the loop: a/b and a are useless,
and the error from a/b/c contains important
information.

The errors are now a bit more verbose than
I'd like but they will suffice for Go 1.11.

$ go get github.com/bradfitz/private/sonos
go get github.com/bradfitz/private/sonos: git ls-remote -q origin in /Users/rsc/pkg/mod/cache/vcs/61e3c76780847e514802ec6af8f940f641c6017f711444f05c59cb17ac46d456: exit status 128:
	remote: Repository not found.
	fatal: repository 'https://github.com/bradfitz/private/' not found
$ go list launchpad.net/gocheck
can't load package: package launchpad.net/gocheck: unknown import path "launchpad.net/gocheck": bzr branch --use-existing-dir https://launchpad.net/~niemeyer/gocheck/trunk . in /Users/rsc/pkg/mod/cache/vcs/f46ce2ae80d31f9b0a29099baa203e3b6d269dace4e5357a2cf74bd109e13339: exec: "bzr": executable file not found in $PATH
$

Fixes #26885.
Fixes #26982.

Change-Id: I2f9cf1853d2d68af18adad668c80513b6ba220d6
Reviewed-on: https://go-review.googlesource.com/129683
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>