Provide CMake target for accelerator support

[devreal]

Description

Software depending on PaRSEC may want to check whether the correct accelerator component has been enabled.

Describe the solution you'd like

PaRSEC could provide PaRSEC::cuda and PaRSEC::hip respectively so that dependents like TTG can check for it.

[therault]

I think the 'proper' CMake way to do that is to expose CUDA, HIP, and other features as COMPONENTS for the PaRSEC package, and the user should use

find_package(PaRSEC COMPONENTS HIP CUDA REQUIRED)

I'm not sure what it means to have a PaRSEC::cuda target, because this target would just be an alias to PaRSEC::parsec?