odb: move computation of normalized objdir into `alt_odb_usable()`

odb: move computation of normalized objdir into `alt_odb_usable()`

The function `alt_odb_usable()` receives as input the object database,
the path it's supposed to determine usability for as well as the
normalized path of the main object directory of the repository. The last
part is derived by the function's caller from the object database. As we
already pass the object database to `alt_odb_usable()` it is redundant
information.

Drop the extra parameter and compute the normalized object directory in
the function itself.

While at it, rename the function to `odb_is_source_usable()` to align it
with modern terminology.

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