I have attempted to install several packages listed at JuliaObserver which are not in the Julia registry but instead are to be downloaded from github. The instructions at JuliaObserver look like this but depending on the particular repository:
Pkg.clone("https://github.com/JuliaDataReaders/DataReaders.jl.git")
These attempts all fail with "UndefVarError: clone not defined".
Yes, Pkg.clone is part of the old API. The new API is documented here https://julialang.github.io/Pkg.jl/v1/api.html
In particular, you get a similar behaviour by
using Pkg
pkg"add <URL or package name>" # If you want to install the package
pkg"dev <URL or package name>" # If you want to develop the package
If the package is registered, you can provide the package name above. If not, you need to provide the full URL to the repo