Shim help, ?, and system.file on package load

These functions are now inserted into a parent environment of global when a package is loaded, instead of existing in the devtools package environment. This is so that the functions are available when a package is loaded, even if devtools itself is not attached.

Another approach is to replace utils::help and utils::? in place. However, it doesn't work to replace base::system.file, because the result there is an infinite loop when it tries to call base::system.file. This is because, unlike utils, base doesn't have a separate package and namespace environment.