debuginfod: fix skipping <built-in> source file

debuginfod: fix skipping <built-in> source file

dwarf_extract_source_paths explicitly skips source files that equal
"<built-in>", but dwarf_filesrc may return a path like "dir/<built-in>".
Check for and skip that case, too.

In particular, the test debuginfod RPMs have paths like this.  However,
the test cases didn't catch this because they have a bug, too: they
follow symlinks, which results in double-counting every file.  Fix that,
too.

Signed-off-by: Omar Sandoval <osandov@fb.com>