This permits switching between the different regex engine modes that
ripgrep supports. The purpose of this flag is to make it easier to
extend ripgrep with additional regex engines.
Closes#1488, Closes#1502
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.
