nmigen-yosys versions are internally inconsistent

[whitequark]

For annoying reasons, the setuptools_scm version of nmigen-yosys does not match the Yosys node version:

 nMigen Yosys 0.9+3527 (PyPI ver 0.9.post4831.dev23, git sha1 a0177569)

We detect features based on the Yosys node version. There is some code that parses the setuptools_scm version in _toolchain.yosys, and it is broken with the currently published nmigen-yosys packages because the versions don't match.

The reason this code parses the Python package version rather than the -V output version is wasmtime's startup latency: I did run python3 -m nmigen-yosys -V at one point, but the performance hit felt unacceptable.

I think the only reasonable way to handle this is to stop using setuptools_scm to get Yosys version and just shell out to git. We must do this before shipping the next release, since our backends already require recent node versions.

[whitequark]

Done and verified on CI.

Since the Yosys node vesion is less than the setuptools_scm version, I had to go and (manually...) delete every nmigen-yosys package ever released. Why delete and not yank? Because of this pip issue.