improve caching of built assets

[smira]

We cache built assets by schematic ID, but for some assets (e.g. installer), some parts of the schematic don't apply (e.g. cmdline).

So if we improve, we can cut down number of assets built by using more specific cache ID.

(This needs to be improved to understand the exact semantics).

[smira]

On a second thought, this might be actually very hard, as the schematic ID is imprinted in most of the assets (as a virtual extension), so each asset is different, even if the output besides that virtual extension is same.