PR gdb/27876 - debuginfod-downloaded source files don't pass proper fullname across...

PR gdb/27876 - debuginfod-downloaded source files don't pass proper fullname across mi / (gdb)info source

Source files downloaded from debuginfod currently use their original DWARF
filename as their "fullname".  This causes a mismatch between the fullname
and the actual location of the source file in the debuginfod client cache.

MI consumers such as VSCode will fail to open debuginfod-downloaded
source files due to this.  Also 'info source' will fail to include the
true paths of these files.

To fix this, use the debuginfod cache path as the fullname for debuginfod-
downloaded source files.