Minor refactor pkg_upgrade tool

Minor refactor pkg_upgrade tool

- Refactor CompareVersion() to return verson compare result directly.
- Change enum type name to use more common abbreviation of "compare".
- Fix using UnzipPkgFromZip() to deliver std::string.
- Remove unnecessary variable initialization.
- Change return type of UnzipPkgFromZip to boolean.
- Replace some command and its arguments into constexpr.
- Extract BackendInvoker constructor with several functions for readability.
- Adjust ordering of include files with alphabetical order.

Change-Id: I4c6f7a8617da1e3fbbf6eca53fe308a8f0b61d8d
Signed-off-by: Junghyun Yeon <jungh.yeon@samsung.com>