[ESI][runtime] Make cmake-distributed header include-able

Currently, https://github.com/llvm/circt/blob/main/lib/Dialect/ESI/runtime/CMakeLists.txt is written in such a way that (i presume) it's expected to be added via `add_subdirectory`. Instead, I think it would be better to have an `esi.cmake` file which can be `include`d, or write the CMake support s.t. we can do `find_library`.

As a minimum, this file needs to be refactored s.t. file-paths are absolute (using `${CMAKE_CURRENT_SORCE_DIR}`) and not relative (e.g. `set(ESIRuntimeSources cpp/lib/Accelerator.cpp ...` -> `set(ESIRuntimeSources ${CMAKE_CURRENT_SORCE_DIR}/cpp/lib/Accelerator.cpp`).