Simplify the processing of skipped source directories

We kept a number of arrays of directory names to keep track of exactly
which directories to look for build.info.  Some of these had the extra
function to hold the directories to actually build.

With the added SUBDIRS keyword, these arrays are no longer needed.
The logic for skipping certain directories needs to be kept, though.
That is now very much simplified, and is made opportunistic.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7558)