- remove unused `BuildEnvironment` parameter: `no_clean`
- move helper to install build requirement to the `BuildEnvironment` class
- simplify a little the 2 different code paths (with and without build isolation)
* fix build environment handling when using PyPy
* do use the build environment for all build commands
* allow installing and building a wheel of a PEP 518 enabled
package without prior installation of setuptools and/or wheels
* fix check for minimum supported requirements for PEP 518 support:
- correctly handle complex requirements
- both setuptools and wheels are needed
Setup build environment before first invocation of setup.py and improve test to actually make check that build requirements are installed upon invocation.