`vars_pull()` should warn on external vectors

[DavisVaughan]

Alternative solution would be to implement eval_pull() and deprecate vars_pull() #189

Here vars_pull() and eval_select() are consistent but only eval_select() warns.

I think vars_pull() should warn too, or we should develop a new eval_pull() and use that instead and fix all these minor issues

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union

d <- data.frame(x = "ex", y = "why")
x <- "y"

pull(d, x)
#> [1] "ex"
select(d, x)
#>    x
#> 1 ex

# Now remove x
d <- select(d, -x)

pull(d, x)
#> [1] "why"
select(d, x)
#> Warning: Using an external vector in selections was deprecated in tidyselect 1.1.0.
#> ℹ Please use `all_of()` or `any_of()` instead.
#>   # Was:
#>   data %>% select(x)
#> 
#>   # Now:
#>   data %>% select(all_of(x))
#> 
#> See <https://tidyselect.r-lib.org/reference/faq-external-vector.html>.
#> This warning is displayed once per session.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#>     y
#> 1 why