DNU restore should still restore unlisted packages

[danroth27]

If the Published date value is 1900-01-01T00:00:00 , then the package is unlisted and it should no longer be considered in dependency resolution when versions are being selected, or during initial installs of packages where users are discovering versions.

However, these "unlisted" versions must still be allowed when the user is restoring packages so that the version doesn't disappear out from under them and break their build.

This feature allows package authors to unlist packages for two reasons:
1.The package is being staged for an upcoming release announcement and they want to list the package at the time of the announcement.
2.The package has a very bad bug and the author doesn't want any new users to discover the version that has the bug.

The logic may be different for the v3 feed and we will need to account for that when we add support for v3.

[davidfowl]

What? #1208

[danroth27]

Yes, did you read Jeff's comment in #1208?

[muratg]

@davidfowl we need to close on the decision here, and perhaps implement it as part of the ongoing nuget work.

[davidfowl]

I'm reverting the whole change. We dont have a difference between restore and install. Thats a much bigger discussion. We can potentially warn on restore of an unlisted package.