Debugger: Fix #9961.

- The previous solution was applying source path mappings too early, with
  the consequence that the file manager would incorrectly adjust some
  relocations due to not being aware of the presence of all the possible
  parent paths yet, leading to the observed "Source not available" behavior
  when restoring some combinations of mappings. We now lazily apply the
  mappings at the point when the source code itself is actually requested
  to be loaded, which doesn't occur until after all image loading, etc.
  has completed, so the information map is complete.