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 <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: Jay Conrod

src/cmd/go/alldocs.go

@@ -29,8 +29,9 @@ import (
 // The caller should report this error instead of continuing to probe
 // other possible module paths.
 //
-// TODO(bcmills): See if we can invert this. (Return a distinguished error for
-// “repo not found” and treat everything else as terminal.)
+// TODO(golang.org/issue/31730): See if we can invert this. (Return a
+// distinguished error for “repo not found” and treat everything else
+// as terminal.)
 type VCSError struct {
 	Err error
 }