How to serve a website by merging multiple base folders?

mod_alias is the answer! [1]

/opt/ina/all/www/
         term/www/
         cli/www/
         pp/www/
         dbg/www/

alias.url += ( "/all" => "/opt/ina/all/www/" )
alias.url += ( "/term" => "/opt/ina/term/www/" )
[...]

/etc/lighttpd/conf-available/50-inaterm.conf:

server.modules += ( "mod_alias" )
alias.url += ( "/term" => "/opt/ina/term/www/" )

Enable a lighttpd configuration:

/usr/sbin/lighty-enable-mod 50-inaterm.conf