Change existing _lpy_ name prefixes to conform to new naming rules

The naming prefix `_lpy_` is currently used throughout the [`loopy.target`](https://github.com/inducer/loopy/tree/master/loopy/target) module. This should be changed to match the variable naming requirements described in the newly created **Identifiers** section of [Reference: Loopy’s Model of a Kernel](https://documen.tician.de/loopy/ref_kernel.html).