We should not assume that the commondir file actually exists. If it
doesn't, then just move on. This otherwise emits an error message when
searching normal submodules, which is not OK.
This regression was introduced in #1446.
Fixes#1520
The top-level listing was just getting a bit too long for my taste. So
put all of the code in one directory and shrink the large top-level mess
to a small top-level mess.
NOTE: This commit only contains renames. The subsequent commit will
actually make ripgrep build again. We do it this way with the naive hope
that this will make it easier for git history to track the renames.
Sigh.
