Log Message:
pkgtools/url2pkg: reorganize the Python implementation

The previous version of the code was largely work in progress. Now the code
has been grouped and sorted. A few bugs have been fixed on the way:

* If a PKGNAME had been added in the interactive editor session, it had
  been overwritten before. This was because of a typo.

* The whole code has been grouped into classes, to clearly show the
  dependencies between the parts.

* Generation of the initial Makefile has been split into smaller methods,
  to make them individually testable and to reduce the scope of the local
  variables.

* When creating a package in a directory pkgsrc/local/*, "local" is not
  used as the primary category of the package.

* GNU configure and other configure scripts are also detected if they are
  not placed directly in WRKSRC.

* Packages that contain *.po files will have USE_PKGLOCALEDIR=yes in the
  package Makefile. Previously, only *.mo or *.gmo files triggered this
  variable.

* When PKGNAME is based on DISTNAME, it is only written to the package
  Makefile if there is an actual prefix or transformation.