Also remove unnecessary namespace qualification.

This allows providing custom argument formatters without relying on
internal APIs (#235).

Minor packaging improvements

This makes sure that the current build directory is not packaged
with the regular source tree when `make package_source` is used.

By default the cmake files are installed to "<prefix>/lib/cmake/cppformat".
This can now be customized using the `FMT_CMAKE_DIR` cache variable.
Valid paths are documented in the cmake manual [1].

e.g., install the cmake files to "<prefix>/share/cmake/cppformat"
```
cmake -DFMT_CMAKE_DIR=share/cmake/cppformat <path-to-source>
```

Tip: Try `make package` to examine the result.

[1] https://cmake.org/cmake/help/v3.0/command/find_package.html