slang: find modules in misnamed files with -v library files

When slang is given a single top file plus -y dir --libext .sv, it only finds a module in a file named after it, so module foo inside bar.sv is UnknownModule and everything below it is missing from elaboration. Passing every other project .sv/.v as -v <file> (a library file: its modules are elaborated only when instantiated; packages and interfaces still load) elaborated a 169-instance SoC from the testbench file alone in about 70 ms. Any remaining errors came from defines that only the project's .f supplied (+define / -D), so prefer a .f whose --top matches the top file, and fall back to the -v scan. Also note that a lookup-by-path API that returns 'deepest known instance + rest of path' will happily answer for paths under the missing module: verify the rest against the instance's own signal list.