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.
Suggest checking that PRs assert that insertions into sets or maps
expected to be adding new values didn't actually just replace some
existing value.
Bug #7048 and its several duplicates would have been caught sooner if
the insertion of the new spill temporary into the `spilled_composites`
table had asserted that there was no existing spill variable for that
expression.
