Revert #196

[sizmailov]

I made a few mistakes that led to this.

1. I did a quick review, overlooking/ignoring suspicious changes
2. I've ignored that feat: ✨ automatically replace invalid enum expressions with corresponding valid expression & import #196 effectively made the parser class 2-pass from 1-pass.

@ringohoffman Please don't take the revert personally.

[sizmailov]

Currently, stubgen does not provide a suitable interface to implement fixups on already parsed modules. Postponing everything to finalize() is working, but it's not the right tool for the job.

[ringohoffman]

It's regrettable that there was a logic error. Are there no architectural changes that you would be happy with to support delayed InvalidExpression fixing?

My original implementation wasn't to do everything in finalize. If the concern is to raise an error when there is >1 possible definition for an enum field used as a default argument, we could just check if we ever try to register the same expression twice, and go back to performing the fixes before finalize.