Don't supply DAP 'path' for non-file shared libraries

Don't supply DAP 'path' for non-file shared libraries

The DAP 'module' event may include a 'path' component.  I noticed that
this is supplied even when the module in question does not come from a
file.

This patch only emits this field when the objfile corresponds to a
real file.

No test case, because I wasn't sure how to write a portable one.
However, it's clear from gdb.log on Linux:

{"type": "event", "event": "module", "body": {"reason": "new", "module": {"id": "system-supplied DSO at 0x7ffff7fc4000", "name": "system-supplied DSO at 0x7ffff7fc4000"}}, "seq": 21}

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30676