Fixes to make Alembic exported config work for OpenEXR 2.x or Imath 3.x

[lgritz]

I believe the way it currently was, the exported AlembicConfig.cmake
was fine for if Imath3 is being used, but for building against older
OpenEXR 2.x, would not work properly.

The change: AlembicIlmBase.cmake sets a variable that reveals whether
Imath3 was found or if we fall back to useing OpenEXR 2.x. Then we use
that to alter what the exported Alembic.config.cmake ought to say in
terms of whether to find_dependency(Imath) or find_dependency(IlmBase).

This extra smidge of complexity can be removed when Imath 3.x is the
true minimum. But given that OpenEXR 2.4 is (unwisely, IMHO) the
choice for VFX Platform 2021 and in the current set of DCC versions, I
believe we need to support OpenEXR 2.4 as a first class citizen for
just a little bit longer.

Signed-off-by: Larry Gritz lg@larrygritz.com

[lgritz]

Aside: I have learned (the hard way) that it's very easy for a project to screw up its generated cmake exported configs and not know it, because it's not actually tested by the project's own CI. What I've found helpful in my other projects is to make sure there is one test as part of the CI somehow that does an install, and then tries to build something trivial against the installed project -- essentially, a test that consumes the project the way a downstream user would, using the exported CMake configs, and 100% independent of the package's own build system.