avoid downloading packages again during installation

[gfursin]

We should provide an option to avoid download packages during package reinstallation.

For now we can do it via some environment key such as SKIP_DOWNLOAD that can be set as "ck install package:... -DSKIP_DOWNLOAD=yes

Then install.sh or install.bat can detect this option and skip download ...

In the future, we should make a smart check of MD5SUM to be sure that package didn't change ...