Detect packages needed by S4 classes

```r
> library(bigmemory)
> X <- as.big.matrix(matrix(1:10, nrow = 2L))
> class(X)
[1] "big.matrix"
attr(,"package")
[1] "bigmemory"
```
In other words, if `X` is a global, we should append:
```r
packages <- c(packages, attr(class(X), "package"))
```

Start by locating the R code that detects globals and assembles the packages vector. Reproduce the bigmemory S4-class example, then verify that a global such as X contributes attr(class(X), "package") to the package list.

Written by the indexing model from the issue text.