Fix deparsing of subs named :::: and ::::::

Fix deparsing of subs named :::: and ::::::

Since a name like :::: is now divided up as ::/:: (since commit
088225f), there can be a package called ‘::’. But %:: returns %main::,
not %main::::. The result is that the main package is searched again
and again recursively and stash_subs hangs.

The easiest fix is to put main:: at the beginning of a substash’s name
when fetching the stash.