Implement Package::implementation() to load implementations of a given type

[thekid]

Implements #15

use com\example\instructions\Instruction;
use lang\reflection\Package;

$package= new Package('com.example.instructions');
$impl= $package->implementation(Instruction::class, ucfirst($name).'Instruction');

[thekid]

Released in https://github.com/xp-framework/reflection/releases/tag/v3.5.0