Autoinstall prompt should appear in more cases

[aplavin]

Currently:

julia> using StatsBase
 │ Package StatsBase not found, but a package named StatsBase is available from a registry. 
 │ Install package?

but

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

I think it should appear whenever the input starts with a using or import statement.

[IanButterworth]

I thought this was handled..