On Windows, we tend to look for .gir files in the $(prefix) directory
indicated by $(prefix)/bin/g-ir-scanner when running g-ir-scanner, so:

*  We add a special case for find_program() to look for g-ir-scanner on
   Windows without any executable extensions, since that is how
   g-ir-scanner is being deployed as a Python script.

*  Update default_search_paths() to first look for the presence of
   g-ir-scanner, and use its "installation prefix" to construct the
   default path where g-ir-scanner looks for the .gir files.

This way, we can make gi-docgen work better on Visual Studio builds
where Python (i.e. the official binary installers from www.python.org)
is generally installed seperately from the $(prefix) of our GNOME stack,
meaning that we can reduce the need to copy the dependent .gir files
from under our $(prefix).