`import lib.X` silently resolves from the library root; SC0109 does not say which path was tried

[czepluch]

A leading lib segment in an import path is special syntax: classifyModulePath (src/Solcore/Frontend/Parser/Decl.hs:96-98) strips it and resolves the rest from the library root. This appears to be undocumented, and the SC0109 diagnostic does not reveal it, which makes the failure genuinely hard to debug when a file physically exists at <root>/lib/X.solc:

repro/
  A.solc
  lib/
    helpers.solc

// A.solc
import std.{*};
import std.dispatch.{*};
import lib.helpers.{*};

contract A {
    constructor() {}
    public function f() -> uint256 { return fortytwo(); }
}

// lib/helpers.solc
import std.{*};

export {
    fortytwo
};

function fortytwo() -> uint256 {
    return uint256(42);
}

sol-core -f repro/A.solc --root repro

->

error[SC0109]: import lib.helpers: file not found
  --> repro/A.solc:3:8
  |
3 | import lib.helpers.{*};
  |        ^^^^^^^^^^^ module reference
help: check the module path or add the missing source file

The file the import names in the obvious reading (repro/lib/helpers.solc) exists; the candidate actually probed is repro/helpers.solc, but the diagnostic never says so. Moving the file to repro/helpers.solc (keeping import lib.helpers) compiles fine.

Seen on current main (1d490d8, nix build).