[PackageGraph] Fix package name validation for product target depende…
…ncies
The package name validation was wrong for product target dependencies
(`.product(name: "<product>", package: "<package>")`) as we were looking
up the package using its identity instead of the name. This completely
breaks package loading in 5.2 if a package wants to use a product from
a dependency that doesn't match the package name of that dependency.
<rdar://problem/59821906>
(cherry picked from commit e3a880b310e5ded7e19a0f8cb3ca91f5e1e777a9)
