[cpackget] Enable full offline installation of local packs

[KeilChris]

Currently, cpackget tries to update the index at least once a day (if the tool is online). This is also done when I try to add a local pack. If the download of the index fails, I have no means to install the pack because it always tries to update the index first.

I would suggest to add a cpackget flag to run it in offline mode/without updating the index first. For example, we could have a no-update or offline command in cpackget.

[jkrech]

Failing to update the index.pidx should print a warning but it should not be fatal. If an index.pidx is available the potentially outdated index.pidx should be used.