Fix leading qualifiers in use-import prefixes

[PLeVasseur]

Summary

• Preserve leading namespace qualifiers when constructing import path prefixes.
• Distinguish empty local prefixes, bare extern-prelude prefixes, and qualified global prefixes.
• Retain qualifiers through nested use trees and reject invalid non-leading or duplicate qualifiers.
• Centralize the empty and bare-:: glob-import restrictions.
• Add the corresponding FLS maintenance accounting.

Closes #711

Reference alignment

• Rust Reference use-declaration syntax
• Rust Reference brace syntax
• Rust Reference extern-prelude restriction
• Follow-up discussion from PR 694