cmd/apidiff: check for package existence
cmd/apidiff: check for package existence

This provides a nicer error experience when providing a package that does not
exist.

Change-Id: I816df12556832af67f432cccd91da2dc5232b798
Reviewed-on: https://go-review.googlesource.com/c/155577
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/exp,golang/exp,golang/exp