Namespace dependencies not required error although I've used Imports in DESCRIPTION

I received the following error when running R CMD check:

Namespace dependencies not required: 'foreach' 'ggplot2' 'magrittr'

I've found a previous question and follow the answer there by making sure that the packages are included in the Imports field of my DESCRIPTION file, but I still received the error.

Here is my DESCRIPTION file

Type: Package
Title: mytitle
Version: 0.0.0.9000
Authors@R: c(
    person("John", "Doe", email = "johndoe@example.com", role = c("aut", "cre")),
    )
Description: More about what it does (maybe more than one line)
    Use four spaces when indenting paragraphs within the Description.
License: What license is it under?
Encoding: UTF-8
LazyData: true
Depends:
    R (>= 3.1)
Imports:
    cowplot (>= 0.9.2),
    data.table (>= 1.11.4),
    doParallel (>= 1.0.11),
    foreach (>= 1.2.0),
    GA (>= 3.1.1),
    ggplot2 (>= 2.1.0),
    hydroGOF (>= 0.3.10),
    magrittr (>= 1.4),
    parallel (>= 3.5.0),
    Rcpp (>= 0.12.16)
RoxygenNote: 6.1.0
Roxygen:
    list(markdown = TRUE)
LinkingTo: 
    Rcpp,
    RcppArmadillo    
Suggests: 
    testthat

And here is my NAMESPACE file created by Roxygen:

export(myfunction)
import(data.table)
import(ggplot2)
importFrom(Rcpp,evalCpp)
importFrom(cowplot,plot_grid)
importFrom(doParallel,registerDoParallel)
importFrom(foreach,"%dopar%")
importFrom(foreach,foreach)
importFrom(hydroGOF,KGE)
importFrom(hydroGOF,NSE)
importFrom(hydroGOF,rmse)
importFrom(hydroGOF,ssq)
importFrom(magrittr,"%>%")
importFrom(parallel,detectCores)
importFrom(parallel,makeCluster)
importFrom(parallel,stopCluster)
useDynLib(mytitle, .registration = TRUE)

Can you please tell me what I missed?