Fix LSP removing static downloads and git refs from lockfile on save

[kylewlacy]

Closes #129

This PR tweaks the load_project function, which is where lockfiles are generated and updated. In this code, we were skipping over Brioche.download and Brioche.gitRef statics in the context of the LSP (where fully_valid is false), which meant that the lockfile that got built by the LSP would never have any download or git ref statics in it. When the LSP saves a file, we would write this recomputed lockfile (in order to update dependencies, which were handled correctly). The updated lockfile would therefore save as if the project had no Brioche.download or Brioche.gitRef statics included, removing them even if they were already present in the existing lockfile.

As a fix, we just copy over the Brioche.download and Brioche.gitRef statics from the existing lockfile if present, even if those statics are no longer used in the project. My thought was that, if someone commented out a Brioche.download, saved the project, then uncommented the Brioche.download, we wouldn't want to get rid of the download static in between. So, this means that the only way to remove Brioche.download / Brioche.gitRef statics from the lockfile is to let the CLI update it (e.g. with brioche check, brioche build, etc)