[develop2] Allow passing pathname to other conanfile.txt

[timwoj]

The develop2 branch appears to require you to have conanfile.py or conanfile.txt in the root of your project. We're only using conan on Windows, so we're storing our conanfile with some other Windows-specific build/CI files. It'd be nice to be able to pass a path to cmake/conan to specify where to look for the conanfile instead of fixing it to a specific location.