cmd/go: query each path only once in 'go get'

If we don't know whether a path is a module path or a package path,
previously we would first try a module query for it, then fall back to
a package query.

If we are using a sequence of proxies with fallback (as will be the
default in Go 1.13), and the path is not actually a module path, that
initial module query will fail against the first proxy, then
immediately fall back to the next proxy in the sequence — even if the
query could have been satisfied by some other (prefix) module
available from the first proxy.

Instead, we now query the requested path as only one kind of path.
If we query it as a package path but it turns out to only exist as a
module, we can detect that as a PackageNotInModuleError with an
appropriate module path — we do not need to issue a second query to
classify it.

Fixes #31785

Change-Id: I581d44279196e41d1fed27ec25489e75d62654e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/189517


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