Allow --project=@scriptdir for easier co-packaging of scripts and p… (

#50864)

…rojects

I have a directory with a number of runnable scripts and a Project.toml
that applies to all of them. I was hoping to have a simple shebang line
for these, but there didn't really seem to be a good way to set the
`--project` without forcing the script to be invoked with a specific cwd
or playing complicated shebang games. With this PR, you can simply
write:

```
#!/usr/bin/env -S julia --project=@scriptdir/..
```

and everything will work out of the box. Perhaps we already have a good
mechanism for this, but if so, I was unable to find it, so in that case,
consider this a documentation issue instead ;).

base/client.jl

@@ -225,6 +225,7 @@ function incomplete_tag(ex::Expr)
 end
 incomplete_tag(exc::Meta.ParseError) = incomplete_tag(exc.detail)
 
+cmd_suppresses_program(cmd) = cmd in ('e', 'E')
 function exec_options(opts)
     quiet                 = (opts.quiet != 0)
     startup               = (opts.startupfile != 2)
@@ -238,10 +239,7 @@ function exec_options(opts)
     repl = !arg_is_program
     cmds = unsafe_load_commands(opts.commands)
     for (cmd, arg) in cmds
-        if cmd == 'e'
-            arg_is_program = false
-            repl = false
-        elseif cmd == 'E'
+        if cmd_suppresses_program(cmd)
             arg_is_program = false
             repl = false
         elseif cmd == 'L'

base/initdefs.jl

@@ -252,6 +252,19 @@ function load_path_expand(env::AbstractString)::Union{String, Nothing}
         env == "@" && return active_project(false)
         env == "@." && return current_project()
         env == "@stdlib" && return Sys.STDLIB
+        if startswith(env, "@scriptdir")
+            if @isdefined(PROGRAM_FILE)
+                dir = dirname(PROGRAM_FILE)
+            else
+                cmds = unsafe_load_commands(opts.commands)
+                if any((cmd, arg)->cmd_suppresses_program(cmd), cmds)
+                    # Usage error. The user did not pass a script.
+                    return nothing
+                end
+                dir = dirname(ARGS[1])
+            end
+            return abspath(replace(env, "@scriptdir" => dir))
+        end
         env = replace(env, '#' => VERSION.major, count=1)
         env = replace(env, '#' => VERSION.minor, count=1)
         env = replace(env, '#' => VERSION.patch, count=1)