Header setter module name should contain "extension"

[jpkrohling]

I should have caught this during the review for the initial version: the header setter extension is currently named "headersetter" in the module and directory name within the "extension" package. Other components do have their types as part of the name, and this is our current convention, so, we should change this. To avoid breaking current users of that extension, we should probably just copy it to the corrected name and deprecate the current one.