cmd/go/internal/web2: make netrc parsing more robust

- Respect the NETRC environment variable if set.

- Ignore lines that contain macro definitions.

- Associate the 'machine' token with only the tokens that follow (not
  precede) it.

Updates #29888
Updates #26232

Change-Id: I3128b7d6da2d6492df7c864e165eea1a27384f0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/161698


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