Avoid AND-ing multi-term specifiers in marker normalization (#4911)
## Summary
Given `python_version != '3.8' and python_version < '3.10'`, the first
term was expanded to `python_version < '3.8'` and `python_version >
'3.8'`. We then AND'd all three terms together. We don't seem to have a
way to differentiate between the terms to AND and the terms to OR in the
normalization code (it all gets flattened together), so instead this PR
expands the expressions at the leaf level and then flattens them at the
level above when appropriate.
Closes https://github.com/astral-sh/uv/issues/4910.
