Postpone importing packaging in `env.py`

[layday]

To simplify bootstrapping we should only import packaging inside the functions where it's used. This would eliminate two dependencies, packaging and pyparsing, which are currently required to bootstrap build.

[FFY00]

To be clear, build can be bootstrapped without isolation, and this is what we do in distro packaging as we supply our own environment.

But yes, this would be a good improvement.

[layday]

I don't know what 'bootstrapped without isolation' means here, but env is imported in main which means you need to have packaging to run build.

[FFY00]

Ah, I missed that then!

[IamRaviTejaG]

Hello @layday @FFY00 👋🏻 . Just tried to address this issue using PR: #395. Please take a look.

[FFY00]

Thank you! I just merged it 😊