Use build from the source dir instead of requiring `build` to be installed.
(can also use the commented out `$python -m flit_core.wheel` alternative, if needed).
Part of the fixes for #10717.
* python3.8: bulk removal of now obsolete "_python38" packages.
Done via scripting. No functional change intended or expected.
Some manual tweaks will follow.
* python3.8: further manual tweaks after bulk-3.8-removal.
Unlike the previous commit with automated changes,
I've tested builds for *these* recipes in beta4 64 bits.
