RepositoriesCollection drops releases of all but the last repository #60

Description

RepositoriesCollection::getReleases() loses releases when it aggregates more than one repository. Two repositories with one release each produce a collection with only the second release.

getReleases() builds the collection from a generator that uses yield from over each repository's releases. Collection::create() turns it into an array with iterator_to_array(), which keeps the keys. Every repository yields keys starting at 0, so a later repository overwrites the releases of an earlier one at the same index.

Today Command::getRepository() passes a single GitHub repository, so the CLI is not affected yet. Any second source added to the collection would silently replace the first one's releases.

Steps to reproduce

1. Remove #[Skip] from RepositoriesCollectionTest::mergesReleasesOfAllRepositories.
2. Run vendor/bin/testo --filter=mergesReleasesOfAllRepositories.

Additional context

The release lookup may be replaced by DLoad in #49, in which case this code goes away.