fix active_shims()

[beanumber]

This currently doesn't work:

https://github.com/baumer-lab/fertile/blob/639fc688b85fc3e700effa3d4f1ea972e16f8564/R/utils.R#L736

When we shim functions, we should add a "package" attribute to the function. Then, any function that is in any environment that has a "package" attribute equal to "fertile" can be safely removed. We can use this functionality in active_shims() and by extension, in unload_shims().

[ambertin]

Got this partially addressed in #104 but still having an issue with some shims. I will try to look into that soon, but for now at least the function is at least mostly functional. Certainly better than it was before.