Moving l10nregistry-rs development from GitHub to mozilla-central

[Daniel Minor]

Hello, Mozilla community - 

l10nregistry-rs is a set of functionality built on top of fluent-rs, a rust implementation of the Fluent localization framework. The l10nregistry functionality is designed to efficiently and effectively solve the complex problem of optimizing the retrieval of localization resources from different locales based on resource availability and on the desired order of precedence among selected locales.  

While  l10nregistry-rs started life on Github, we believe the only consumer of this product is Firefox, and keeping it in a separate version control system has introduce significant friction to its development, with pain points including integration overhead, vendoring-version mismatches and problems with developer environment setup, to name a few.

With that in mind, we intend to migrate the l10nregistry-rs crate into the mozilla-central repository. This will make the process of maintaining and modifying the l10nregistry-rs library simpler, testing more rigorous and change management easier and more rigorous.

Note that this change doesn't bring fluent-rs along with it, which may still require occasional maintenance and re-vendoring.  Fluent already has a well-established ecosystem with community users, downstream consumers and fluent-related tooling across various languages, so while many of the same costs apply, they are incurred far less often and fluent-rs and the community around it are well-served by having it remain on Github.

We intend to make this change on March 25, 2022. 

Thank you,

Daniel Minor