Added a special-case so we always find an implementation of Python

Added a special-case so we always find an implementation of Python

If we can't find a distribution package for Python, add the version of
Python currently interpreting 0install as a candidate.

This is a temporary hack so that programs can state their dependency on Python
explicitly without breaking on platforms where we don't support the native
package manager.

Many feeds currently assume that Python must be installed, since 0install itself
depends on it, and use a #! line to run it. However, this doesn't work on
Windows or on Linux systems where "python" is now Python 3.