reftable/blocksource: consolidate code into a single file

reftable/blocksource: consolidate code into a single file

The code that implements block sources is distributed across a couple of
files. Consolidate all of it into "reftable/blocksource.c" and its
accompanying header so that it is easier to locate and more self
contained.

While at it, rename some of the functions to have properly scoped names.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>