Move Exporter from lib/ to dist/Exporter/

Exporter has been considered dual life, upstream blead, since commit
6295adb525682844 (Sep 2006), but it was not moved to dist/ in 2009 with
the other dual-life modules because it was not possible to disentangle it
from the early stages of the build bootstrapping.

The build bootstrapping is now sufficiently simplified that it's possible
to move it to dist/