`Tplyr:::modify_nested_call()` fails if Tplyr is not loaded

[Michael Stackhouse]

This arose from reverse dependency failures in safetyCharts. The following code will fail if Tplyr isn't loaded:

Tplyr::tplyr_table(safetyData::adam_adsl, TRT01P) %>%
  Tplyr::add_layer(
    Tplyr::group_count(RACE) 
  ) %>%
  Tplyr::build()
Error in as.environment(pos) : 
  no item called "package:Tplyr" on the search list

This is because of a call to objects("package:Tplyr") which is looking at namespaces on the search path. This can be avoided by using getNamespaceExports("Tplyr") instead.