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.

Tracks rust-lang#711.

Reference alignment

• Rust Reference use-declaration syntax:
  https://doc.rust-lang.org/reference/items/use-declarations.html#r-items.use.syntax
• Rust Reference brace syntax:
  https://doc.rust-lang.org/reference/items/use-declarations.html#r-items.use.multiple-syntax
• Rust Reference extern-prelude restriction:
  https://doc.rust-lang.org/reference/items/use-declarations.html#r-items.use.restrictions.extern-prelude
• Follow-up discussion from PR 694:
  document missing import restrictions rust-lang/fls#694 (comment)

The FLS models brace grouping through explicit recursive prefix construction rather than the Reference's prose description. This is a structural difference with no intentional behavioral deviation.

The changes are limited to Rust 2021 semantics. The Rust 2015 interpretation of leading :: is excluded.

Glob-import restrictions are centralized in fls_iNUBX5fJAI1N; its macro-consumption allowance intentionally applies to both empty and bare-:: glob prefixes because macros may consume those token forms before import resolution rejects them.

Testing

• ./make.py --clear
  • Passed with no warnings or errors.
• ./make.py --check-links
  • Passed: 31 HTML files, 16,985 links checked, zero errors.
• Rust 1.95.0 positive matrix under edition 2021
  • All six valid declarations accepted.
• Rust 1.95.0 negative matrix under edition 2021
  • All six invalid declarations produced the expected diagnostics.
• git diff --check upstream/main...HEAD
  • Passed.