Revert "Avoid installing URL dependencies multiple times."

This reverts commit 644c2ff3e3d9c93764f7045762477f48864d64a7.

It's a good patch, and none of us caught the problem.  However, this
introduces an arguably surprising new behavior, since `npm link -f` and
`npm install -f` will now reinstall ALL deps, no matter what.
v1.3.17