[Build] Use CMakeDeps Conan generator

Whilst attempting to write a test executable with an embedded Python
interpreter for OpenAssetIO#739, it turned out that the old problem reported in
OpenAssetIO#528 was still present.

To recap, ultimately (via circuitous logic) every CMake target created
by the Conan pybind11 package links to `libpython` (the
`pybind11::embed` target is the only one that should). This means that
Python extension modules (`pybind11::module`s) link to `libpython`,
causing linker errors at runtime or even segfaults due to ODR
violations.

As of conan-io/conan-center-index#13283 this bug
was fixed upstream. However, we still see it because the bug was not
fixed for the deprecated `cmake_find_package` generator, which we use.

It has not affected us so far because we haven't added in the
`Development.Embed` component to our `find_package(Python` call. As soon
as we add that component (e.g. for adding the aforementioned Python unit
test) we see this problem.

So modernise our `conanfile.py` to use the non-deprecated `CMakeDeps`
generator.

`CMakeDeps` insists on having the `build_type` setting available, so add
that.

Unfortunately `CMakeDeps` is designed to install dependencies with the
same configuration (Release, Debug, etc) as the parent project, which is
usually undesirable - we want Release versions of dependencies whilst
we develop the project in Debug mode. This is logged upstream in
conan-io/conan#11607, where a workaround is
posted, which is used here (see code comments).

Signed-off-by: David Feltell <david.feltell@foundry.com>

resources/build/conanfile.py

@@ -1,19 +1,37 @@
-from conans import ConanFile, CMake, tools
+from conans import ConanFile, tools
+from conan.tools.cmake import CMakeDeps, CMakeToolchain
 
 
 class OpenAssetIOConan(ConanFile):
-    generators = (
-        # Generate a CMake toolchain preamble file `conan_paths.cmake`,
-        # which augments package search paths with conan package
-        # directories.
-        "cmake_paths",
-        # Generate `Find<PackageName>.cmake` finders, required for
-        # various public ConanCenter packages (e.g. pybind11) since they
-        # disallow bundling of `<PackageName>Config.cmake`-like files in
-        # the package.
-        "cmake_find_package",
-    )
-    settings = "os"
+    settings = "os", "build_type"
+    # Generate a CMake toolchain preamble file `conan_paths.cmake`,
+    # which augments package search paths with conan package
+    # directories.
+    # TODO(DF): This is deprecated and should be swapped for the
+    # CMakeToolchain generator, but that is not yet compatible with the
+    # cpython recipe (it does not specify `builddirs` so is not added to
+    # the CMAKE_PREFIX_PATH).
+    generators = "cmake_paths"
+
+    def generate(self):
+        """
+        This function is called at the end of a `conan install` and is
+        responsible for generating build toolchain helpers.
+        """
+        # Generate `<PackageName>Config.cmake` files for each
+        # dependency, which CMake's `find_package` will locate and
+        # parse.
+        # Note: we override the configuration and re-run multiple times
+        # to allow us to use a different configuration for dependencies
+        # than we use for the main project. See
+        # https://github.com/conan-io/conan/issues/11607#issuecomment-1188500937
+        deps = CMakeDeps(self)
+        deps.configuration = "Release"
+        deps.generate()
+        deps.configuration = "Debug"
+        deps.generate()
+        deps.configuration = "RelWithDebInfo"
+        deps.generate()
 
     def requirements(self):
         # CY2022