Properly index debug info from gnu_debugaltlink file

[osandov]

.gnu_debugaltlink is generated by dwz, which is used by Linux distributions to deduplicate debugging information between multiple binaries. For example, coreutils binaries share a gnu_debugaltink file, as do the python executable and libpython. In fact, in the Python case, almost all of the debugging information is in the gnu_debugaltlink file.

844d828 added just enough support so that we could load debug files using .gnu_debugaltlink. But, we don't index any information from the alt file, so type and object lookups from the alt file still fail.

Supporting this will enable debugging Linux distribution packages, like Python, that use .gnu_debugaltlink.