Add new symbol directory layout by build-id `/xx/xxxxxxxx.debug`.

[piwicode]

This protocol is already supported by various tools and lldb. e.g.: https://github.com/llvm-mirror/lldb/blob/d01083a850f577b85501a0902b52fd0930de72c7/source/Symbol/LocateSymbolFile.cpp#L311

Among others it is used by Fuchisa OS builds.

This change was unit tested with fetch_test.go. It was manually tested with a FuchsaOS build by pointing pprof at $FUCHSIA_HOME/out/default/.build-id

[codecov-commenter]

Codecov Report

Merging #724 (2d56bd5) into main (70bd9ae) will increase coverage by 0.00%. The diff coverage is n/a.

:exclamation: Current head 2d56bd5 differs from pull request most recent head 314f5ea. Consider uploading reports for the commit 314f5ea to get more accurate results

@@           Coverage Diff           @@
##             main     #724   +/-   ##
=======================================
  Coverage   64.67%   64.68%           
=======================================
  Files          45       45           
  Lines        6602     6603    +1     
=======================================
+ Hits         4270     4271    +1     
  Misses       1884     1884           
  Partials      448      448           

Impacted Files	Coverage Δ
...c/github.com/google/pprof/internal/driver/fetch.go	63.01% <0.00%> (+0.12%)	:arrow_up:

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[aalexand]

@piwicode CI fails due to wrong source code formatting.

[piwicode]

This should be fine now. Thanks.