* Fixes#5754
Previously when RepoPath.repo_for_pkg was invoked with a string,
it did not check if the string included a namespace. Any
namespace-qualified package provided as a string would not be found
(at which point the behavior was to return the highest-precedence
repository).
* handle nested namespaces for packages specified as strings in repo_for_pkg
* add preliminary repository tests
* add test which replicates #5754
* refactor repo tests with fixtures
* define repo_path equivalent at test-level scope for repo tests
* add tests for unknown namespace/package
* rename fixture function (no longer prefixed with 'test_')
dcdd16b35e