radlink: honor explicit library paths for bare DEFAULTLIB names

[honkstar1]

Problem

An explicit C:/sdk/lib/provider.lib input can be followed by /DEFAULTLIB:provider.lib, either on the command line or in an object's directives. If C:/sdk/lib is not on LIBPATH, radlink searches for the bare name again and warns that it cannot find a library it has already queued by full path. LLD recognizes the explicit input's basename.

Fix

Keep a separate, case-insensitive basename table for explicitly supplied libraries. Consult it for default-library requests after the existing extension normalization and /NODEFAULTLIB check. Do not strip directory components from a request and do not mix these aliases into the path-identity table.

This preserves distinct explicitly named libraries and path-qualified defaults with identical basenames. Genuinely missing libraries still produce diagnostics; their existing warning/error severity is unchanged.

Standalone on dev@0e2b7ed2: 17 implementation lines plus one regression test in the existing test file. No dependency on #842, #892, or #932.

Validation

• Fresh standalone Win64 Clang/LLVM 20.1.8 release and debug builds: 27 library/weak-symbol/unresolved-symbol tests passed in each, zero failures/crashes/skips.
• The new test covers relative/absolute explicit paths, case and extension normalization, object directives, two distinct same-basename libraries, path-qualified defaults, and real missing-library diagnostics.
• The same regression test fails on the pre-fix development binary with warning 059; it passes with LLD 20.1.8. The omitted-extension check is RAD-specific because LLD checks visited names before adding .lib.
• The identical production fix in the combined development build passed its broader release/debug subsets (162 each, 12 existing MSVC-invoking exclusions). On the original SDK-module response file it removed the warning and produced byte-identical DLL/PDB output.

No MSVC compiler, linker, or assembler was invoked locally. The standalone tests require no proprietary SDK or compressed-object support. Full CI is separate from these focused checks.