Set package_folder attribute of ConanFile instance before build

[robertmrk]

Some of the C++ projects which use CMake as a build system have an install target. Their listsfiles already contain the definition of what and how should be installed. Defining the installation steps of the project in the package method of the ConanFile duplicates this information. Ideally I would like to maintain the code for installation at only one place.
So for projects with an install target, in the package method of the ConanFile we can build the install target instead of defining again which files need to be copied.
For this packaging method to work, the CMAKE_INSTALL_PREFIX of the project should be set at the CMake configuration step, when the build method is called, but at the moment the package_folder attribute of the ConanFile instance is set only before the package method is called.
This PR fixes this issue by setting the package_folder attribute before the build method is called. So in the build method, the project can be configured with the right CMAKE_INSTALL_PREFIX and by building the install target in the package method all the required files of the project would get installed in the package directory.

def build(self):
    cmake = CMake(self.settings)
    self.run("cmake %s %s -DCMAKE_INSTALL_PREFIX=%s" % \
        (self.conanfile_directory, cmake.command_line, self.package_folder))
    self.run("cmake --build . %s" % cmake.build_config)

def package(self):
    self.run("cmake --build . --target install")

[memsharded]

Please don't remove this. There might be some users using it.

[memsharded]

Seems a very good idea, nice and practical solution. Just if you could leave the package_folder before the package() too, as some users might use that value there too. Also, if you have a working example, would you mind to copy and paste some snippets and open a docs issue? Or just open the issue. I would like to add this to the docs. Thanks very much for your contribution!

[robertmrk]

I reverted the changes in the create_package function.
OK, I'll open an issue for the docs soon.
You're welcome. I'm glad I could help.

[memsharded]

Merged, it will be released in 0.10. Thanks again!