cmd/go: make 'go get' match patterns against packages, not modules
This is a follow-up to CL 174099, fixing an important TODO.
The 'go help modget' documentation will be clarified in anotehr CL,
pending further discussion.
When invoked without -m, 'go get' will no longer match arguments
containing "..." against module paths. If a module's path matches a
pattern but no packages within the module match the pattern, the
module should not be upgraded. For example, if
golang.org/x/tools/playground and golang.org/x/tools are separate
modules, and only golang.org/x/tools is in the build list,
'go get golang.org/x/tools/playground/...' should add
golang.org/x/tools/playground to the build list and leave
golang.org/x/tools alone.
Updates #26902
Change-Id: I2bd18c7950db1aa7bd8527210c1baf2c7d174375
Reviewed-on: https://go-review.googlesource.com/c/go/+/176578
Run-TryBot: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>