pack-objects: move static inline from a header to the sole consumer

pack-objects: move static inline from a header to the sole consumer

Move the code that is only used in builtin/pack-objects.c out of
pack-objects.h.

This fixes an issue where Solaris's SunCC hasn't been able to compile
git since 483fa7f42d9 (t/helper/test-bitmap.c: initial commit,
2021-03-31).

The real origin of that issue is that in 898eba5e630 (pack-objects:
refer to delta objects by index instead of pointer, 2018-04-14)
utility functions only needed by builtin/pack-objects.c were added to
pack-objects.h. Since then the header has been used in a few other
places, but 483fa7f42d9 was the first time it was used by test helper.

Since Solaris is stricter about linking and the oe_get_size_slow()
function lives in builtin/pack-objects.c the build started failing
with:

    Undefined                       first referenced
     symbol                             in file
    oe_get_size_slow                    t/helper/test-bitmap.o
    ld: fatal: symbol referencing errors. No output written to t/helper/test-tool

On other platforms this is presumably OK because the compiler and/or
linker detects that the "static inline" functions that reference
oe_get_size_slow() aren't used.

Let's solve this by moving the relevant code from pack-objects.h to
builtin/pack-objects.c. This is almost entirely a code-only move, but
because of the early macro definitions in that file referencing some
of these inline functions we need to move the definition of "static
struct packing_data to_pack" earlier, and declare these inline
functions above the macros.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>