This avoids printing several 'Found:' messages during configure, and
also avoids doing several searches for the same binary. This is already
done by the interpreter for `find_program` calls from build files.
Also move it to the module-wide __init__.py file so it can be used by
other modules as-needed.
Also use it for g-ir-scanner where it was missed in one place, also fix
exception name in the same place.
It is often useful to be able to check if a specific object is of a type
defined in a module. To that end, define all such targets in
modules/__init__.py so that everyone can refer to them without poking
into module-specific code.
Nirbheek Chauhan