If we are using a remote source (a URL), and the user did not specify
the executable file to use, then don't try to use a local source.
This was misbehaving because the local symbolizer will not fail
if there is any memory map available, but the presence of a memory map
does not ensure that the files and symbols are actually available.

We still need a pprof testsuite.

Fixes #16159.

Change-Id: I0250082a4d5181c7babc7eeec6bc95b2f3bcaec9
Reviewed-on: https://go-review.googlesource.com/24464
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>