PackIndexWriter: create interface to write indexes

PackIndexWriter: create interface to write indexes

PackWriter assumes that the primary index goes to a file in a well-known
format. This cannot accomodate implementations in other storages or
formats (e.g. in a database).

Create an interface to write the index (PackIndexWriter). This interface
will be implemented by the existing pack index writer classes
(PackIndexWriterV1 etc.).

As the "PackIndexWriter" name was used by the previous superclass of the
file writers, we rename that class to "BasePackIndexWriter".

Change-Id: Ia7348395315e458fc7adc75a8db5dcb903e2a4a1