Move HttpContextAccessor from .Internal namespace

[davidfowl]

If users want to use this API they need to call

services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();

Today this requires adding using Microsoft.AspNet.Http.Internal;

[dougbu]

Suggest we take this further and move everything or almost everything our own repos depend on from .Internal into a public namespace. Right now Antiforgery, BasicMiddleware, CORS, Diagnostics, Entropy, Hosting, Identity, ... all use HttpAbstraction's .Internal namespaces. And all such namespaces defined in this repo except Microsoft.AspNetCore.Http.Authentication.Internal have references from other repos.

[muratg]

@dougbu Good to close?

[dougbu]

No. I'm doing more of this in concert w/ the #592 fix at @davidfowl's urging.

[dougbu]

Taking this one because we're tying fix together w/ #592 i.e. will do one set of reaction PRs.

[dougbu]

Will move the following types to Microsoft.AspNetCore.Http. All of these are currently used in other aspnet repo's product code.

• DefaultHttpContext
• FormCollection (though not mainstream, necessary for some scenarios e.g. handling a form-URL-encoded response as in aspnet/Security)
• HeaderDictionary
• HttpContextAccessor (remains the most important of these changes because it's necessary for any use of the IHttpContextAccessor feature e.g. in aspnet/Identity)
• HttpContextFactory

Will also move files around in the repo so the remaining Microsoft.AspNetCore.Http.Internal classes are in an Internal sub-folder.