Allow symlinks in `create_package`

[antass]

This issue came up when using rrtools::use_compendium which is a wrapper for usethis::create_package. Below is the issue I reported to rrtools:

When initiating a new compendium with:

rrtools::use_compendium("~/path/to/new/compendium")

where ~/path/to/new is a symlink to /some/other/path, rrtools throws an error:

> rrtools::use_compendium("~/path/to/new/compendium")
Error: '/home/nfs/user.name/path/to/new' is not a directory.

However, replacing symlink with full path works:

rrtools::use_compendium("/some/other/path/compendium")

[hadley]

Reprex:

library(fs)

base <- dir_create(file_temp())
base_a <- dir_create(path(base, "a"))
base_b <- link_create(base_a, path(base, "b"))

usethis::create_package(path(base_b, "c"))
#> Error: '/tmp/RtmpzLKzKT/file286037f9d19f/b' is not a directory.

^{Created on 2019-10-17 by the reprex package (v0.3.0)}