Remove Vector from the default LESS import path

When Ia193571a added vector to core for VectorBeta's benefit it introduced a
regression that broke all skins that used imports like 'variables.less' which
happen to match a filename used by Vector.

This is a short-term fix that allows us to release 1.23 without this regression
by moving the bug from core to the VectorBeta extension.
It needs to be be backported to 1.23.
And we will still need one or two long-term fixes for the bug.

Bug: 60368
Bug: 64595
Dependency: I8ee928a8f60751f72f2a37250d274cf55bce6ee7
Change-Id: Iee47bdc23630e02ccfcbd28496ec5268892eb629