Replace rlang::is_installed() with base::requireNamespace()

[jdblischak]

Closes #246

This PR removes the direct dependency on {rlang} by replacing rlang::is_installed() with base::requireNamespace(quietly = TRUE). I was originally going to create an internal function is_installed(), but even that seems uncessary since this code is always hidden from the user in the examples and vignettes.

I tested locally that requireNamespace() behaves as required:

remove.packages("dplyr")
library("simtrial")
requireNamespace("dplyr", quietly = TRUE)
.Last.value
## [1] FALSE
example("get_cut_date_by_event")
devtools::check(force_suggests = FALSE) # Sets _R_CHECK_FORCE_SUGGESTS_⁠ to FALSE
## N  checking package dependencies (11s)
##    Package suggested but not available for checking: 'dplyr'

xref: #245, Merck/gsDesign2#307, #195 (review), #201

[yihui]

Looks good to me!

[nanxstats]

Great! Thanks for restoring the dependency state to where it should be.