go/doc: reuse import name logic for examples

go/doc: reuse import name logic for examples

Examples resolved imports based on just the path name,
but this doesn't work for go-name or modules on v2+.

Updates #12794
Fixes #45110
Fixes #56740
Fixes #62059

Change-Id: I8c71b1e5311df04bccbdf319d759d3176a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/728280
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>