exportGlobals() should make sure global actually lives in package before dropping

[HenrikBengtsson]

Internal exportGlobals() should make sure global object actually lives in package before dropping it. This would for instance avoid dropping FUN in if it is assigned as FUN <- base::mean in, say, the global environment.

Currently, exportGlobals() is a bit naive and just trusts the environment of the object;

  pkgs <- packagesOf(globals)
  ## Drop all globals which are already part of one of
  ## the packages in 'pkgs'.  They will be available
  ## when those packages are attached.
  pkgsG <- sapply(globals, FUN=function(obj) {
    environmentName(environment(obj))
  })
  keep <- !is.element(pkgsG, pkgs)
  globals <- globals[keep]

As a start it should make sure the name (by names(globals)) matches.

Related to: Issue HenrikBengtsson/globals#9