bug#19390: 25.0.50; `package-activate' is too slow

[Dmitry Gutov]

Before c13baa10d55ec863d3ceaea48c6b2959ece98198, `package-initialize'
takes ~100 ms on my machine, with 92 packages.

After this revision, it takes ~7.7 seconds.

Provided there's no obvious low-hanging fruit for performance
improvement, we should at least avoid exercising the new logic when
Emacs is just launching. There's simply nothing to reload yet.

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2014-12-14 on axl
Repository revision: eaf25ad549dc5a9b26089f588e0a80268708a3d1
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:     Ubuntu 14.04.1 LTS