Pkg3: running a package

[Over](https://discourse.julialang.org/t/distinguishing-projects-from-packages/153) in discourse it was discussed to distinguish between "runnable packages" (called projects in that thread) and "library packages" (called packages). The [suggestion](https://discourse.julialang.org/t/distinguishing-projects-from-packages/153/35?u=mauro3) which gathered the most likes was not to distinguish between projects and packages, but instead to "standardize where to put runnable scripts into packages as we know them now. Say a folder `run/` or `scripts/` and the main program would be `run/main.jl`. Pure "Projects" would have an empty `src/` folder and full `run/` folder and vice versa (most would have a bit of both). Similar to `Pkg.test("SomePkg")` we could have a `Pkg.run("SomePkg")` to `run run/main.jl`." Also a command-line option could be good, say `julia --run SomePkg`.

(I haven't followed this Julep too closely, please close this issue if this is in it already. Or let me know if this should be posted over in Julia itself.)

Start by reading the linked Discourse discussion and the Julep's current status. Then inspect the existing Pkg commands and Julia command-line options mentioned in the issue. Done means reaching an agreed design for package-run conventions and the proposed Pkg.run or julia --run interface.

Written by the indexing model from the issue text.