Move use of ``__file__`` to be local to template access

Move use of ``__file__`` to be local to template access

Moved the use of the ``__file__`` attribute at the base of the Alembic
package into the one place that it is specifically needed, which is when
the config attempts to locate the template directory. This helps to allow
Alembic to be fully importable in environments that are using Python
memory-only import schemes.  Pull request courtesy layday.

Partially-fixes: #648
Closes: #651
Pull-request: https://github.com/sqlalchemy/alembic/pull/651
Pull-request-sha: c30299dbc3499c59e81602d91deb3b03166b4583

Change-Id: I2607031d80c418ce4d7e779eeb42c57a53a72ce0