loader: build a loose RomFS once and only when it is read

Every loader of an extracted title built the whole RomFS from its romfs/
directory in its constructor, though most are made only for metadata
(the game list, title lookups), and each build keeps an object per file:
title 1's 206k files left four copies alive, ~0.6 GB of the heap. The
RomFS is now built on first read, and loaders of the same directory share
one build while any of them holds it.

(cherry picked from commit be47212cac130e6a895423644751c0525dc6f169)

0 files changed, 0 insertions, 0 deletions