internal/frontend: replace LegacyPackage in TestCreatePackage
internal/frontend: replace LegacyPackage in TestCreatePackage

For golang/go#39629

Change-Id: Ide026122af1d6e4d49acad35ea5c8405f454bbed
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/263209
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite