Prevent Yarn from rewriting the lockfile when switching between online ⇄ offline (#51)

* Create 0000-offline-resolution-field.md

* Update 0000-offline-resolution-field.md

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Author: arcanis

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+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: (leave this empty)
+- Yarn Issue: (leave this empty)
+
+# Summary
+
+When enabling the offline mirror, Yarn updates the lockfile by stripping the registry URL from its `resolved` fields. This RFC aims to simplify this process by making such an update unneeded.
+
+Related issues: https://github.com/yarnpkg/yarn/issues/393 / https://github.com/yarnpkg/yarn/issues/394
+
+Tentative implementation: https://github.com/yarnpkg/yarn/pull/2970/
+
+# Motivation
+
+Yarn currently has two different type of values for the lockfile `resolved` fields:
+
+  - When online, they're in the form `${source}/${name}-${version}.tar.gz#${hash}`
+  
+  - But when offline, they're instead `${name}-${version}.tar.gz#${hash}`
+
+The current reasoning (or at least side effect) seems to be that it allows the fetch process to refuse installing things from the network when running under the `--offline` switch (and to always fetch things from the network otherwise instead of looking into the offline mirror). Unfortunately, such a separation also makes it harder to switch between working with a remote registry and an offline repository (for example, dev environments might not need the offline repository, but under the current design they can't do without).
+
+Because of these reasons, it would be best for the `resolved` field to contain the same informations during both online and offline work, *as long as the files we fetch are the expected ones* (ie. their hashes match).
+
+# Detailed design
+
+I suggest the following:
+
+  - Adding the `${source}` part to the `resolved` field even when offline
+
+# Drawbacks
+
+Nothing should break. More iterations will be required to address the other issues raised in the previous iterations of this document.
+
+# How We Teach This
+
+This change is quite transparent, since it's unlikely the users will ever want to update the yarn.lock file manually.
+
+# Alternatives
+
+  - Instead of adding the package registry to each `resolved` field, we could remove it instead.