Don't tell people to use Pkg to install Pkg

[oxinabox]

Occasionally one ends up in a situation where Pkg is not available
This results in the rather nonsensical error message:

julia> using Pkg
ERROR: ArgumentError: Package Pkg not found in current path.
- Run `import Pkg; Pkg.add("Pkg")` to install the Pkg package.

How dare the REPL mock me like that?

We could display a number of useful and/or humours messages here, like checking if @stdlib is JULIA_LOAD_PATH
or commentary about being up shit creek without a paddle, etc.
right now for initial PR I just said nothing.

(One way to replicate this error message is to set the environment variable JULIA_LOAD_PATH="some nonsense value" before starting julia. Another is to be building julia from source and have Pkg be in a broken state where it can't precompile. I am sure there is another where you prune stdlibs for your use-case)