Inconsistent behavior in predictor selection with vs. without population scaling

See how the predictor selection varies when we use "analogous" workflows with vs. without population scaling (based on the example for `step_population_scaling`):

``` r
library(epiprocess)
#>
#> Attaching package: 'epiprocess'
#> The following object is masked from 'package:stats':
#>
#> filter
library(epipredict)
#> Loading required package: parsnip
jhu <- epiprocess::jhu_csse_daily_subset %>%
dplyr::filter(time_value > "2021-11-01", geo_value %in% c("ca", "ny")) %>%
dplyr::select(geo_value, time_value, cases)

pop_data = data.frame(states = c("ca", "ny"), value = c(20000, 30000))

r <- epi_recipe(jhu) %>%
step_population_scaling(df = pop_data,
df_pop_col = "value",
by = c("geo_value" = "states"),
cases, suffix = "_scaled") %>%
step_epi_lag(cases_scaled, lag = c(7, 14)) %>%
step_epi_ahead(cases_scaled, ahead = 7, role = "outcome") %>%
step_epi_naomit()

f <- frosting() %>%
layer_predict() %>%
layer_threshold(.pred) %>%
layer_naomit(.pred) %>%
layer_population_scaling(.pred, df = pop_data,
by = c("geo_value" = "states"),
df_pop_col = "value")

wf <- epi_workflow(r, parsnip::linear_reg()) %>%
fit(jhu) %>%
add_frosting(f)

names(coef(extract_fit_engine(wf)))
#> [1] "(Intercept)" "cases_scaled" "lag_7_cases_scaled"
#> [4] "lag_14_cases_scaled"

r2 <- epi_recipe(jhu) %>%
step_epi_lag(cases, lag = c(7, 14)) %>%
step_epi_ahead(cases, ahead = 7, role = "outcome") %>%
step_epi_naomit()

f2 <- frosting() %>%
layer_predict() %>%
layer_threshold(.pred) %>%
layer_naomit(.pred)

wf2 <- epi_workflow(r2, parsnip::linear_reg()) %>%
fit(jhu) %>%
add_frosting(f2)

names(coef(extract_fit_engine(wf2)))
#> [1] "(Intercept)" "lag_7_cases" "lag_14_cases"
```

^{Created on 2023-04-07 by the [reprex package](https://reprex.tidyverse.org) (v2.0.1)}

In this case, the workflow with scaling has a lag-0 predictor, while the one without does not. If, instead, we were to use `lag = c(0, 7, 14)`, then both versions would have a lag-0 predictor, but the one with scaling would have two copies of it, and might cause issues for some engines.