This will make it easier for contributors to understand the file layout,
at the cost of said layout containing several more nested directories.
I will personally appreciate not having to remember to look for
`root.rs` instead of `main.rs`.
I also renamed the test targets so that they do not *all* share the
superfluous suffix “-test” (test targets live in a different namespace
than other target types and packages, so the name can presume that it
is always known that they are tests).
The Naga snapshot data sets `naga/tests/{in,out}` have been left in
their original positions.
