setup: install translation files as package data
Remove the `install_package_data' subclass of `install_data' and use
the `package_data' functionality provided by distutils instead. As
package data must be located within the package directory, the data
files are now generated in the build directory.
To simplify the functionality of this change, the top-level `doc' and
`templates' directories have been moved into the `mercurial' package
directory.
