Modernize Enzyme dependency in cmake

[PhilipFackler]

Summary

Treat Enzyme like a true cmake dependency

Rationale

CMake code for linking with Enzyme is complicated (thus error-prone) and duplicated across all dependent libraries. It would be better to define the build requirements in one place and let dependent libraries just use the target.

Description

• Update FindEnzyme.cmake to be a true "find module"
• Update link code for library targets to use enzyme target instead of itemizing build requirements

Additional information

No response

[nkoukpaizan]

At the time, it wasn't obvious which plugin (ClangEnzyme, LLVMEnzyme or LLDEnzyme) and compile flags to use. It may be a good time to revisit now that the old Enzyme examples have been removed (#494).