Add a dnu command to clear the HTTP cache

[DamianEdwards]

dnu caches results for the various categories of HTTP requests it makes (e.g. feed check, list package, v3 feed check, etc.) for various amounts of time.

While it's possible to run a dnu restore at any time with the cache disabled via --no-cache, there isn't a way to clear the cache completely, other than just deleting the folder in the user profile.

Propose we add a command to clear the HTTP cache, e.g. dnu --clear-cache. Might be nice to support clearing it only for specified feeds too, e.g. dnu --clear-cache https://www.myget.org

This hit me today as we found an issue in the NuGet v3 feed root index.json file which is now fixed. However, dnu caches the request to that file for a week, so customers will need to manually delete the cache folder to see the change in the meantime.

[glennc]

Assigning to beta8 for at least the clear-cache command, we will talk about clearing a specific URL but we might punt that part to beta8

[MisterJames]

@glennc I could take this one on if you like.

1. Can I assume that a raw dnu --clear-cache would clear out the DnuFolderPath.HttpCacheDirectory folder entirely? Or should it clear the list_ and nupkg_ but preserve the dir structure and leave the index_json.dat?
2. When clearing for a specific feed, would https://www.myget.org match all entries in the cache with the start of that domain? Or would it be only on direct matches such as https://www.myget.org/F/aspnetvnext/api/v2/ or https://www.myget.org/F/roslyn-nightly/?

[glennc]

I had assumed it would nuke the whole folder. But I am not familiar enough with the entire mechanism to say for certain. @davidfowl @anurse @lodejard What do you guys think?

[MisterJames]

@glennc Whole folder for the base scenario, yes. But @DamianEdwards had said:

Might be nice to support clearing it only for specified feeds too, e.g. dnu --clear-cache https://www.myget.org

[muratg]

Done: 3d0edc0