app support in Pkg (#3772)

This provides initial support for defining "apps" in Julia which can be run from the terminal and installed via Pkg.

docs/src/apps.md

@@ -0,0 +1,59 @@
+# [**?.** Apps](@id Apps)
+
+!!! note
+    The app support in Pkg is currently considered experimental and some functionality and API may change.
+
+    Some inconveniences that can be encountered are:
+    - You need to manually make `~/.julia/bin` available on the PATH environment.
+    - The path to the julia executable used is the same as the one used to install the app. If this
+      julia installation gets removed, you might need to reinstall the app.
+    - You can only have one app installed per package.
+
+Apps are Julia packages that are intended to be run as a "standalone programs" (by e.g. typing the name of the app in the terminal possibly together with some arguments or flags/options).
+This is in contrast to most Julia packages that are used as "libraries" and are loaded by other files or in the Julia REPL.
+
+## Creating a Julia app
+
+A Julia app is structured similar to a standard Julia library with the following additions:
+
+- A `@main` entry point in the package module (see the [Julia help on `@main`](https://docs.julialang.org/en/v1/manual/command-line-interface/#The-Main.main-entry-point) for details)
+- An `[app]` section in the `Project.toml` file listing the executable names that the package provides.
+
+A very simple example of an app that prints the reversed input arguments would be:
+
+```julia
+# src/MyReverseApp.jl
+module MyReverseApp
+
+function (@main)(ARGS)
+    for arg in ARGS
+        print(stdout, reverse(arg), " ")
+    end
+    return
+end
+
+end # module
+```
+
+```toml
+# Project.toml
+
+# standard fields here
+
+[apps]
+reverse = {}
+```
+The empty table `{}` is to allow for giving metadata about the app but it is currently unused.
+
+After installing this app one could run:
+
+```
+$ reverse some input string
+emos tupni gnirts
+```
+
+directly in the terminal.
+
+## Installing Julia apps
+
+The installation of Julia apps are similar to installing julia libraries but instead of using e.g. `Pkg.add` or `pkg> add` one uses `Pkg.Apps.add` or `pkg> app add` (`develop` is also available).

ext/REPLExt/completions.jl

@@ -156,6 +156,23 @@ function complete_add_dev(options, partial, i1, i2; hint::Bool)
     return comps, idx, !isempty(comps)
 end
 
+# TODO: Move
+import Pkg: Operations, Types, Apps
+function complete_installed_apps(options, partial; hint)
+    manifest = try
+        Types.read_manifest(joinpath(Apps.app_env_folder(), "AppManifest.toml"))
+    catch err
+        err isa PkgError || rethrow()
+        return String[]
+    end
+    apps = String[]
+    for (uuid, entry) in manifest.deps
+        append!(apps, keys(entry.apps))
+        push!(apps, entry.name)
+    end
+    return unique!(apps)
+end
+
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