build: assume vendor tools are in the PATH and remove automatic sourcing, source...

build: assume vendor tools are in the PATH and remove automatic sourcing, source and toolchain_path parameters.

Automatic sourcing was not consistent between build backends (and only really supported by ISE/Vivado)
and had no real additional value vs the complexity needed to support it. Now just assume required vendor
tools are in the PATH.

This also removes distutils dependency.