Open a path-based source once, process-wide, synchronously. Returns the shared instance (opening it on the
first call), or undefined if the path needs async opening (a .br/.gz bundle -- use getSharedSigSource).
Throws only if a sync-openable source fails to open.
Open a path-based source once, process-wide, synchronously. Returns the shared instance (opening it on the first call), or
undefinedif the path needs async opening (a.br/.gzbundle -- use getSharedSigSource). Throws only if a sync-openable source fails to open.