Can I create a package with friendly modules

[Tejas]

On Sunday, 12 June 2022 at 05:05:46 UTC, forkit wrote:
> Is it possible to create a package.d, consisting of (for 
> example), two modules, where each module can access private 
> declarations within each other.
>
> In essence, declaring 'a module level friendship', or a kind of 
> 'extended module' if you want.
>
> I might still want to add another module to the package, that 
> is NOT part of that friendship between those other two modules, 
> but is otherwise related to the solution.

You can use `package(qualifiedIdentifier)` to satisfy this to a 
greater extent

Directory structure:
```sh

src
|
|--- main_file.d
|
|---parent
       |
       |--- driver.d
       |
       |--- package.d
       |
       |--- thing
              |
              |--- package.d
              |
              |--- first.d
              |
              |--- second.d
              |
              |--- third.d
```

Code :
`src/main_file.d`:
```d
import parent;

void main()
{
	func();
}
```

`src/parent/package.d`:
```d
module parent;

public import
	driver;
```

`src/parent/driver.d`:
```d
module parent.driver;

import thing;

void func()
{
	S s;			// from third.d
	auto t = first.a;	// from second.d
	auto l = second.dbl;	// from first.d
}
```

`src/parent/thing/package.d`:
```d
module parent.thing;

public import
	first,
	second,
	third;
```

`src/parent/thing/first.d`:
```d
module thing.first;

import second;

static this()
{
	a = second.g; // can also access symbols within neighbouring 
modules
}
package(parent):
int a;
int b;
string c;
```

`src/parent/thing/second.d`:
```d
module thing.second;

package(parent):

int g;
char p;
double dbl;
```

`src/parent/thing/third.d`:
```d
module thing.third;

package(parent):

struct S
{
	int a;
	char c;
	string s;
}

private S private_struct; // can't access via parent package, 
since marked private
```

You run it via:
`dmd -i -I=./parent -I=./parent/thing main_file.d`
(I'm pretty sure it'll look far prettier using `dub`)

Here, all the symbols that are within package `thing` are visible 
to package `parent` as well, but not visible to any other 
package/module