Fix dirichlet_conrey import

Harald added an lmfdb prefix to the imports of the dirichlet_conrey
module, but these shouldn't be there. This is a standalone module
which needs to be separately installed.
Fix some more imports

Changed a bunch of occurances of base.* to lmfdb.base.* and changed from
imports of lmfdb.base to 'import lmfdb.base as base'.
More updates for Harald's modulization.

Also, a Z-function fix is hidden here. The latest sage patch which
implements the Hardy Z-function does it correctly, so that Z(t)
corresponds to the L-function on the half line. This updates our code
to take this into account.