Render module tree per package in the content page (#1492)

* Render module tree per package in the content page

When rendering content page for multiple packages it is useful to split
the module tree per package.  Package names in this patch are inferred
from haddock's interface file names.

* Write PackageInfo into interface file

To keep interface file format backward compatible, instead of using
`Binary` instance for `InterfaceFile` we introduce functions to
serialise and deserialise, which depends on the interface file version.