Suppose I have a `build.lua`, a `foo.dtx` and a `foo.ins` with the directory structure as this:

```
.
├── build.lua
├── foo.dtx
└── foo.ins
```

and the the contents as follows:

### `build.lua`

```
module = "foo"
```

### `foo.dtx`

```
%    \begin{macrocode}
%<*pkg>
\def\foo{foo}
%</pkg>
%    \end{macrocode}
%    \begin{macrocode}
%<*def>
\def\foo{foo}
%</def>
%    \end{macrocode}
```

### `foo.ins`

```#
\input l3docstrip.tex
\usedir{tex/latex/\jobname}
\generate{%
  \file{\jobname.sty}{\from{\jobname.dtx}{pkg}}%
}
\usedir{tex/latex/\jobname/def}
\generate{%
  \file{\jobname.def}{\from{\jobname.dtx}{def}}%
}
\endbatchfile
```

I am facing difficulty in understanding the following:

1. When I run `latex foo.ins`, my directory looks like:
   ```
   .
   ├── foo.def
   ├── foo.dtx
   ├── foo.ins
   └── foo.sty
   ```
   (Note that I skipped `build.lua` and `foo.log` as they were irrelevant.)
   
   I don't understand the role of `\usedir{tex/latex/\jobname/def}` in this case. When I specifically mention the path (`./def` in this case, see line number 6 from the ins file) in which I want to see a particular file (`foo.def` in this case), why is the non-existent path not created? I expect the path to be created and the then the specific file to be generated there. Instead, the file is installed in the same directory where the .sty is generated. To me, this seems quite counter intuitive. 
1. When I run `l3build install`, what all from the `.ins` file is relevant? If the content of the `.ins` file is relevant, then maybe fixing 1 would solve the issue for `l3build` too, but suppose it isn't, then the question is how do I instruct `l3build` to create the structure I need. I tried adding the following to `build.lua` with no success.
   ```
   maindir = "."
   texmfdir = maindir .. "def/"
   sourcefiles = { "*.def","*.sty","*.dtx","*.ins" }
   installfiles = sourcefiles
   ```
   This creates all the necessary files in the `build/` directory as well as in the `texmf/` directory, but still the desired directory structure is not obtained. At the end everything is boiled down to a flat structure which I don't want.
1. Along with `latex foo.ins` and `l3build install`, I also want my regression testing to work with the new directory structure. So how do I set things up for that? Also, if I create a CTAN zip with `l3build ctan`, that should also produce the correct directory structure.