gnu: Use 'this-package' as a native input instead of 'self-native-input?'.

gnu: Use 'this-package' as a native input instead of 'self-native-input?'.

* gnu/packages/file.scm (file)[self-native-input?]: Remove.
[native-inputs]: New field.
* gnu/packages/guile.scm (guile-1.8)[self-native-input?]: Remove.
[native-inputs]: New field.
(guile-2.0)[self-native-input?]: Remove.
[native-inputs]: Add THIS-PACKAGE when (%current-target-system) is true.
* gnu/packages/ncurses.scm (ncurses)[self-native-input?]: Remove.
[native-inputs]: Add THIS-PACKAGE when (%current-target-system) is true.
* gnu/packages/python-xyz.scm (python-file)[self-native-input?]: Remove.
[native-inputs]: New field.