Make it easier for the community to find packages after the move

[analogrelay]

A few suggestions that came up from the discussion thread on the package move for ensuring people can find which packages live where more quickly. This issue is tracking doing that work. Some of it should be automatic, but we should validate it:

1. Ensure the Source Repository metadata in the packages is correct so that NuGet.org links will work

2. Add a list of all Microsoft.Extensions.* packages that used to exist in this repo linking to where they now live. This isn't necessarily designed to be a constantly-updated list as new packages are added, but should try to cover the ones that used to be maintained in this repo.

[analogrelay]

@bradygaster you were going to write up a README change right?

[bradygaster]

@anurse - yes the readme you authored in the announcements page was great IMO - anyone have any objection to us linking to that, or do folks think we need to essentially copy that content here?

[analogrelay]

Let’s do both. I think a link is easy for people to miss and if it’s copy-pasted it is Ctrl-F friendly. Maybe a copy-paste of the list and a link “for the complete announcement, see aspnet/Announcements#NNNN”