Handle cyclic imports and sources

[EagleoutIce]

Even though we do not expect them to be frequent in reality, we need a way of handling cyclic and diamond like sources. While for source only the cyclic import:

# a.R
source("b.R")

# b.R
source("a.R")

by expecting to support library and other "loading mechanisms" it may be best to allow a handler if a file is requested an nth-time.