Adds support for cwd in the git protocol

[arcanis]

What's the problem this PR addresses?

The git: protocol supports cloning workspaces from a given repository, but only if the project to install is at the root of the repository. It may not be the case when the project contains libraries for multiple languages.

How did you fix it?

A new cwd parameter lets users pick a subpath to use when packing the project. It can be used on its own, or in tandem with the workspace specifier.

Checklist

• [x] I have read the Contributing Guide.

• [x] I have set the packages that need to be released for my changes to be effective.

• [x] I will check that all automated PR checks pass before the PR gets reviewed.