go/internal/gcimporter: propagate errors from FindPkg

go/internal/gcimporter: propagate errors from FindPkg

Previously, FindPkg returned the empty string as a sentinel value,
causing Import to collapse all errors to "can't find import".

(See also https://go.dev/wiki/CodeReviewComments#in-band-errors.)

For #61064.

Change-Id: I21f335d206308b44fe585619e00782abb0b65a94
Reviewed-on: https://go-review.googlesource.com/c/go/+/507360
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>