Fix issues around frontend path discovery of externally built extension modules

[dime10]

A few issues around path discovery of externally built extension modules:

1) Runtime nanobind module discovery repeatedly appends to path

There are two locations where this happens for the runtime registry module:

catalyst/frontend/catalyst/jax_primitives.py

Line 485 in c6f3180

sys.path.append(get_lib_path("runtime", "RUNTIME_LIB_DIR"))

and

catalyst/frontend/catalyst/compiler.py

Line 92 in c6f3180

sys.path.append(rt_lib_path)

Because these lines are run every-time functionality from the module is used, the system path would get polluted with more and more duplicate entries. This doesn't break anything but is not ideal.

A solution could perform this task once upon import for example, similar to how we handle the mlir_quantum module:

catalyst/frontend/catalyst/__init__.py

Lines 55 to 62 in c6f3180

	if not INSTALLED:
	import os
	default_bindings_path = os.path.join(
	os.path.dirname(__file__), "../../mlir/build/python_packages/quantum"
	)
	if os.path.exists(default_bindings_path): # pragma: no cover
	sys.path.insert(0, default_bindings_path)

The thing to keep in mind is that these modules may not exist in some contexts, in particular for a CI docs build which uses only the Python code. So code importing such modules would fail in this context. Typically, we resolve this with mocking the modules in the docs config:

catalyst/doc/conf.py

Lines 88 to 90 in c6f3180

	MOCK_MODULES = [
	"mlir_quantum",
	"mlir_quantum.runtime",

2) The fix should consider a similar module that will be merged soon from the MLIR component to make sure all such modules are handled uniformly

See #2259 which introduced the new module.

3) The path mlir_quantum should consider custom build directories when not in INSTALLED mode.

See also #2713 (comment) for a discussion of this.