Update manifests by default

[christopher-dG]

This finds all Manifest.toml files, activates their environments, and runs Pkg.update.
There is a platform-specific find invocation in there, I suppose it should probably go away if we have something like #85. It could be implemented instead with walkdir or Glob.jl (although vtjnash/Glob.jl#19 is needed).

[DilumAluthge]

FYI we should do the package operations in a separate Julia process.

So for example, you'd have something like this:

function update_manifests()
    list_of_manifest_files = blah blah blah # use walkdir
    for f in list_of_manifest_files
        @info "Updating manifest: $f"
        run(`$(Base.julia_cmd()) -e 'import Pkg; Pkg.activate("$(f)")'; Pkg.update()`)
    end
end

[DilumAluthge]

Also, we'll need at least some kind of test

[christopher-dG]

Can you explain why this needs to happen in a new process?

[DilumAluthge]

Here are some snippets from Kristoffer in a Slack conversation in #pkg-usage on Thursday:

Sounds like you should spawn a new julia process with --project=example/A/ instead. Because if you load stuff in one environment and then change environment and load other things, strange stuff can happen.

Loading stuff from different environments in one process is just not great.

Well, you shouldn't really load things from different subprojects in the same process either.

[christopher-dG]

Works for me 😄

[DilumAluthge]

Here's a question for you.

If both path/Project.toml and path/Manifest.toml exist, which is a better idea?

1. cd into path and run Pkg.instantiate(); Pkg.update()
2. cd into path, delete the path/Manifest.toml, and then run Pkg.instantiate(); Pkg.update()

[christopher-dG]

Hm, I'm not sure if there's a difference. But AFAIK, just doing update will update the manifest in the right way.