Remove fields from local manifest (#493)

Remove fields from local manifest (#493)

* Merge Alire.Releases.TOML_IO into Alire.Releases

Now that Alire.Releases is no longer Preelaborable, there is no reason for the
separation.

* Do not emit/load `origin` for local manifests

This information is only relevant for index manifests, so better not to confuse
users with fields that shouldn't be there.

* Make optional some fields in local manifests

Fields related to packaging can be required at the time of `alr publish`,
whereas they're not important for users not intending to package (yet).

* Testsuite adjustments

* Setup function for tests with init'ed crates

* Testsuite: verify no `[origin]` in local manifests