content: update context article to use stdlib import path

Updated context article to use stdlib import path.
Updated package-names article to reflect that context
is now in stdlib, and added x/time/rate as an example
for a longer package name.

Fixes golang/go#15562

Change-Id: I58d495527037f5201897825ca11f484561997277
Reviewed-on: https://go-review.googlesource.com/29759
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed