cmd/go: make get -u upgrade only modules providing packages

Currently, 'go get -u' upgrades modules matching command line
arguments and any modules they transitively require. 'go get -u' with
no positional arguments upgrades all modules transitively required by
the main module. This usually adds a large number of indirect
requirements, which is surprising to users.

With this change, 'go get' will load packages specified by
its arguments using a similar process to other commands
('go build', etc). Only modules providing packages will be upgraded.

'go get -u' now upgrades modules providing packages transitively
imported by the command-line arguments. 'go get -u' without arguments
will only upgrade modules needed by the package in the current
directory.

'go get -m' will load all packages within a module. 'go get -m -u'
without arguments will upgrade modules needed by the main module. It
is equivalent to 'go get -u all'. Neither command will upgrade modules
that are required but not used.

Note that 'go get -m' and 'go get -d' both download modules in order
to load packages.

Fixes #26902

Change-Id: I2bad686b3ca8c9de985a81fb42b16a36bb4cc3ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/174099


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>