u/danielw/dm-1395

Creating PR for reviewing DM-1395
I thought a while about this. It would have saved me some work to do it that way. However, it is inconsistent with the idea we've been sticking with for code layout--that is, packages are logical groupings meant to make it easy for humans to navigate, understand, and manipulate the code. This is why we have broken from the standard sconsutils layout and placed .h next to .cc, and python/*py next to those, organized on a per-module basis. Bill suggested that separating by language was a bit artificial, and I think most of us agreed that modules should be unified. That philosophy still seems worth keeping, so I went through the trouble of crafting the capability. It's possible that the css layout is the first of its class rather than a single exception.