Override library() and require() calls.

[coatless]

The present use of on package event handling needs to be changed since the search state must only be changed after or before the package is loaded.

That is, stopping the package mid-load gives problematic errors:

library(*)
#> Error in as.environment(lib.pos) : invalid 'pos' argument

require(*)
#> Failed with error:  'invalid 'pos' argument'

https://github.com/coatless/watcher/blob/dd33b2435d2a498c9bb63bb54e5dd95b6e2da998/R/watchers.R#L35-L40