Contain the dependency on IREE

[ftynse]

Multiple packages transitively include IREE packages, which significantly increases load time and clashes with other MLIR-based imports. Specifically, importing wave_lang.kernel or any subpackage (because of wave_lang/kernel/__init__.py importing a lot of stuff) currently loads IREE even if no compilation happens.

[ftynse]

The remaining annoyance is pytest that seems to be visiting all files in the package and may transitively import IREE because some of the tests test the compilation. E.g., https://github.com/iree-org/wave/pull/615/files#diff-b8a504d463b25d1146eef60fa6d9c0211fa4edfaebe5d597a7652753d13bf8bb would fail if managed by pytest. We should find a way to contain that visitation, e.g., by staging compiler-dependent tests in a separate directory and requiring two invocations of pytest.