[IFRT] Rename XlaProgram to HloProgram, move it from PJRT-IFRT to IFRT.
[IFRT] Rename XlaProgram to HloProgram, move it from PJRT-IFRT to IFRT.

HLO programs represented in MLIR are independent of PJRT or XLA as a compiler, so they don't belong in PJRT-IFRT.

Also rename the Python binding API from get_xla_program to get_hlo_program, but this API is not used yet.

Cleanup; no functional changes intended.
