New build target methods

The build target object now supports the following two functions, to ensure feature compatebility with external program objects:

• found(): Always returns true. This function is meant to make executables objects feature compatible with external program objects. This simplifies use-cases where an executable is used instead of an external program.

• path(): (deprecated) does the exact same as full_path(). NOTE: This function is solely kept for compatebility with external program objects. It will be removed once the, also deprecated, corresponding path() function in the external program object is removed.