Look for debug symbols in binutils-type build-id directories

[rjogrady]

Please answer these questions before submitting your issue. Thanks!

What version of pprof are you using?

b4f4732 (2018-11-28)
If you are using pprof via go tool pprof, what's your go env output?
N/A
If you run pprof from GitHub, what's the Git revision?
N/A

What operating system and processor architecture are you using?

Linux 64-bit

When symbolizing, pprof already looks for elfs in a variety of places, such as
$PPROF_BINARY_PATH/<build_id>, $PPROF_BINARY_PATH/<build_id>/<binary_name>,
etc.

The debug info extracted from debian packages is stored in files called usr/lib/debug/.build-id/build_id[:2]/build_id[2:].debug
e.g.
/usr/lib/debug/.build-id/2b/c3a6cf803d6dc59ec11f2773e1b4a795008dbf.debug is the debug elf corresponding to an elf with build-id 2bc3a6cf803d6dc59ec11f2773e1b4a795008dbf

So for symbolizing when we have the debug symbols separate from the stripped shared library, it would be helpful if pprof could try to locate these as well, e.g by searching for
$PPROF_BINARY_PATH/ab/cdefgh.debug

[aalexand]

@rjogrady - please feel free to send a PR.

[aalexand]

I think this is fixed by #724.