small fixes for qisys.command.find_program

[dmerejkowsky]

• On Windows, we should look for files with extension first, or even never look for file without extensions. Often they are scripts with shebangs (#!) that Windows cannot handle (examples are mvn and gcovr)
• It would be convenient to insert paths to be searched directly in find_program

Right now you have to write something like:

my_dir = "C:\\foo\\bin"
import qisys.envsetter
# Prepend a dir to environ["PATH"]
env_setter = qisys.envsetter.EnvSetter()
env_setter.prepend_to_path(my_dir)
env = env_setter.get_build_env()
foo_exe = qisys.command.find_program("foo", env=env)

It would be better to have:

my_dir = "C:\\foo\\bin"
foo_exe = qisys.command.find_program("foo", paths=[my_dir])