.packages returns information about package availability.
.packages(all.available = FALSE, lib.loc = NULL)
logical; if TRUE return a character vector
of all available packages in lib.loc.
a character vector describing the location of R
library trees to search through, or NULL. The default value
of NULL corresponds to .libPaths().
A character vector of package base names, invisible unless
all.available = TRUE.
.packages() returns the names of the currently
attached packages invisibly whereas
.packages(all.available = TRUE) gives (visibly) all
packages available in the library location path lib.loc.
For a package to be regarded as being available it must have valid
metadata (and hence be an installed package). However, this will
report a package as available if the metadata does not match the
directory name: use find.package to confirm that the
metadata match or installed.packages for a much slower
but more comprehensive check of available packages.