Lock file location and add test coverage and doc for LoadProject

[tro3]

Spinning off a specific case from #41.

While adding a unit test for Ctx.LoadProject, I found that in the case where the lock file is in the same directory as the manifest but the dirtree had to be ascended to find it, the lock will not be loaded. If the lock file is always located alongside the manifest, this is a bug and I can fix alongside the test additions. Just let me know.

Also, I assume we want the doc in the docstring, or is there another location? Thx.

[sdboyer]

Yup, that's definitely a bug - the manifest and lock should always be colocated in the same directory. (I think about it as the manifest defining the location of the root, then the lock and vendor being derived from there)

Also, I assume we want the doc in the docstring, or is there another location? Thx.

I was going to say that we might want it in a separate place somewhere, but now that LoadProject is exported (after #178), I think the docstring is plenty for now.