dpkg: Always reset want status when installing a package

Move pkg_set_want() call from wanttoinstall() to the call site in
process_archive(), so that the status-want change affects all ‘true’
return values and not just on “selecting previously unselected package”.

This also makes wanttoinstall() a function that just performs a decision
without acting on it, matching its name.

Closes: #162541