Refactoring L0 runner and gpu dialect

Writing down requirements for refactoring igpu dialect and L0 runner. Goal of this document is to keep track of tasks for refactoring code base to avoid duplicate effort.

- level zero runner and igpu work would continue on main branch until most of the following requirements/features are ready on refactor branch.
- Some of these may be applied to main branch during that period.
- Any task with a checkmark against it indicates it is completed in the `refactor` branch.
- Following list assume "dpcomp" project specific passes and dialects will be redone in `refactor` branch.

Also ran some experiments with https://github.com/silee2/mlir-extensions/blob/cleanup/mlir/CMakeLists.txt to filter out files not need to for level zero runner usage case.

- [ ] #236
- [x] #237
- [x] Remove dpcomp-opt and add imex-opt by using proper dialect and pass registration mechanism #233
- [ ] #238
- [ ] #239
- [ ] #240
- [ ] #241
- [ ] #242
- [ ] #243
- [x] Remove local FileCheck util and use -DLLVM_INSTALL_UTILS flag when building LLVM
- [x] Add IMEXConfig
- [ ] #244
- [x] #245
- [x] #246
- [ ] #247
- [ ] #248
- [ ] #249
- [ ] Add support for various build situations: Ex) as an LLVM external project (LLVM_EXTERNAL_${project}_SOURCE_DIR), as part of an external build, standalone build with local LLVM/MLIR. See mlir-hlo project and #211 for reference.
- [ ] #250
- [ ] #251
- [ ] #252
- [ ] #253
- [ ] #254