cmd/go: fix 'go test' and 'go fmt' with files outside a module

Use the actual loader result in findModule instead of making
assumptions about nesting in the build list.
As a side-effect, this produces much clearer error messages for
packages that (for one reason or another) failed to load.

Adjust the package and module path outside a module to
"command-line-arguments". That string already appears in the output of
a number of (module-mode and GOPATH-mode) commands for file arguments,
and as far as I can tell operation outside a module is currently the
only case in which the module path of a package is not actually a
prefix of the import path.

Fixes #28011
Fixes #27099
Fixes #28943
Updates #27102
Updates #28459
Updates #27063

Change-Id: I61d5556df7b1b7d1efdaffa892f0e3e95b612d87
Reviewed-on: https://go-review.googlesource.com/c/153459


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>