The signatures of functions in containerd's errdefs packages are very
similar to those in our own, and it's easy to accidentally use the wrong
package.

This patch uses a consistent alias for all occurrences of this import.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

This reverts the changes made in 2a9c987e5a72549775ffa4dc31595ceff4f06a78, which
moved the GetHTTPErrorStatusCode() utility to the errdefs package.

While it seemed to make sense at the time to have the errdefs package provide
conversion both from HTTP status codes errdefs and the reverse, a side-effect
of the move was that the errdefs package now had a dependency on various external
modules, to handle conversio of errors coming from those sub-systems, such as;

- github.com/containerd/containerd
- github.com/docker/distribution
- google.golang.org/grpc

This patch moves the conversion from (errdef-) errors to HTTP status-codes to a
 api/server/httpstatus package, which is only used by the API server, and should
not be needed by client-code using the errdefs package.

The MakeErrorHandler() utility was moved to the API server itself, as that's the
only place it's used. While the same applies to the GetHTTPErrorStatusCode func,
I opted for keeping that in its own package for a slightly cleaner interface.

Why not move it into the api/server/httputils package?

The api/server/httputils package is also imported in the client package, which
uses the httputils.ParseForm() and httputils.HijackConnection() functions as
part of the TestTLSCloseWriter() test. While this is only used in tests, I
wanted to avoid introducing the indirect depdencencies outside of the api/server
code.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>