Use-import prefix construction does not preserve leading `::`

Problem

The FLS use-import model constructs an import path prefix primarily from path segments. A leading ::, however, is a namespace qualifier rather than a path segment, so the construction in fls_WAA4WmohGu6T does not clearly retain it.

The grammar permits a use-import prefix consisting only of ::, while the related prefix definition and construction describe prefixes in terms of a leading simple path or its path segments. This leaves the FLS unable to rigorously distinguish these Rust 2021 cases:

use {self as this_module};      // Valid: empty local prefix.
use ::{self as root};           // Invalid: imports the bare extern-prelude root.
use ::std::{self as std_alias}; // Valid: global path through the extern prelude.

In particular:

• An empty prefix with no leading :: should resolve from the current module.
• A bare :: prefix denotes the extern-prelude root and cannot be imported.
• A prefix such as ::std must retain ::; otherwise it can become indistinguishable from locally resolved std.
• Prefix construction through nested use trees must retain :: while ensuring it remains at the start of the fully combined path.

This problem predates #694. That PR exposes it by adding a restriction that refers directly to an import path prefix consisting only of ::.

Expected result

The FLS should model the complete use-import prefix, including any leading namespace qualifier, and align the grammar, prefix definitions, construction rules, and legality rules.

The resulting model should account for:

// Valid.
use {self as this_module};
use ::std::{self as std_alias};
use {::{std::mem as m}};

// Invalid.
use ::{self as root};
use ::*;
use ::{*};
use std::{::{std::mem as m}};
use ::{::{std::mem as m}};

This issue is limited to the inherited leading-::/import-prefix model. The keyword-import restrictions added by #694 and unrelated self wording cleanups do not need to be reopened here.

References

• Follow-up discussion from #694: https://github.com/rust-lang/fls/pull/694#discussion_r3547319628
• Rust Reference, use-declaration restrictions: https://doc.rust-lang.org/reference/items/use-declarations.html#r-items.use.restrictions.extern-prelude
• Rust Reference, brace syntax: https://doc.rust-lang.org/reference/items/use-declarations.html#r-items.use.multiple-syntax

Start at fls_WAA4WmohGu6T and inspect the grammar, prefix definitions, construction rules, and legality rules for use-import paths. Compare the model with the Rust Reference restrictions and the valid and invalid examples in this issue. Done means leading :: is retained through nested use trees and the listed cases are distinguished correctly.