`niv verify` command to verify already tracked packages

[nlewo]

Sometimes, the url of a package can disappear or the content can be updated in place (the sha is then no longer valid). It would be nice to have a niv verify [PACKAGE] [--update-sha] command to verify the url and sha are still valid. The --update-sha option could be use to ask niv to update the sha if it differs. Some tools call this kind of command fsck.

[andir]

How is this different from nix-build sources.nix and thus downloading all the files? Can you give more information on how you would use this?