bpo-33053: -m now adds *starting* directory to sys.path (GH-6231) (#6236)

bpo-33053: -m now adds *starting* directory to sys.path (GH-6231) (#6236)

Historically, -m added the empty string as sys.path
zero, meaning it resolved imports against the current
working directory, the same way -c and the interactive
prompt do.

This changes the sys.path initialisation to add the
*starting* working directory as sys.path[0] instead,
such that changes to the working directory while the
program is running will have no effect on imports
when using the -m switch.

(cherry picked from commit d5d9e02dd3c6df06a8dd9ce75ee9b52976420a8b)