Request: use current directory be used as the default package to build

[halpo]

It's slightly annoying to set the package at the beginning of every session. Might I suggest that the current directory be used as the default. This requires very minor changes to the as.package function. I thought this so minor a change that there was no need to include it in a formal pull request.

#' Coerce input to a package.
#' 
#' Possibile specifications of package:
#' \itemize{
#'   \item path
#'   \item name (lookup in .Rpackages)
#'   \item package object
#' }
#' @param x object to coerce to a package
#' @export
as.package <- function(x = NULL) {
  if (is.null(x)) {
    x <- get_last_package()
  }
  if (is.null(x)) {
    message("package not set, checking '.'")
    x <- '.'
  } 
  else if (is.package(x)) 
    return(x)

  path <- find_package(x)
  if (is.null(path)) {
    stop("Can't find package ", x, call. = FALSE)
  }

  pkg <- load_pkg_description(path)
  set_last_package(pkg)
}