Add initial draft of src vs flat layout discussion (#1150)

Author: pradyunsg

source/discussions/index.rst

@@ -12,3 +12,4 @@ specific topic. If you're just trying to get stuff done, see
    pip-vs-easy-install
    install-requires-vs-requirements
    wheel-vs-egg
+   src-layout-vs-flat-layout

source/discussions/src-layout-vs-flat-layout.rst

@@ -0,0 +1,79 @@
+=========================
+src layout vs flat layout
+=========================
+
+The "flat layout" refers to organising a project's files in a folder or
+repository, such that the various configuration files and
+:term:`import packages <Import Package>` are all in the top-level directory.
+
+::
+
+    .
+    ├── README.md
+    ├── noxfile.py
+    ├── pyproject.toml
+    ├── setup.py
+    ├── awesome_package/
+    │   ├── __init__.py
+    │   └── module.py
+    └── tools/
+        ├── generate_awesomeness.py
+        └── decrease_world_suck.py
+
+The "src layout" deviates from the flat layout by moving the code that is
+intended to be importable (i.e. ``import awesome_package``, also known as
+:term:`import packages <Import Package>`) into a subdirectory. This
+subdirectory is typically named ``src/``, hence "src layout".
+
+::
+
+    .
+    ├── README.md
+    ├── noxfile.py
+    ├── pyproject.toml
+    ├── setup.py
+    ├── src/
+    │    └── awesome_package/
+    │       ├── __init__.py
+    │       └── module.py
+    └── tools/
+        ├── generate_awesomeness.py
+        └── decrease_world_suck.py
+
+Here's a breakdown of the important behaviour differences between the src
+layout and the flat layout:
+
+* The src layout requires installation of the project to be able to run its
+  code, and the flat layout does not.
+
+  This means that the src layout involves an additional step in the
+  development workflow of a project (typically, an
+  :doc:`editable installation <setuptools:userguide/development_mode>`
+  is used for development and a regular installation is used for testing).
+
+* The src layout helps prevent accidental usage of the in-development copy of
+  the code.
+
+  This is relevant since the Python interpreter includes the current working
+  directory as the first item on the import path. This means that if an import
+  package exists in the current working directory with the same name as an
+  installed import package, the variant from the current working directory will
+  be used. This can lead to subtle  misconfiguration of the project's packaging
+  tooling, which could result in files not being included in a distribution.
+
+  The src layout helps avoid this by keeping import packages in a directory
+  separate from the root directory of the project, ensuring that the installed
+  copy is used.
+
+* The src layout helps enforce that an
+  :doc:`editable installation <setuptools:userguide/development_mode>` is only
+  able to import files that were meant to be importable.
+
+  This is especially relevant when the editable installation is implemented
+  using a `path configuration file <https://docs.python.org/3/library/site.html#index-2>`_
+  that adds the directory to the import path.
+
+  The flat layout would add the other project files (eg: ``README.md``,
+  ``tox.ini``) and packaging/tooling configuration files (eg: ``setup.py``,
+  ``noxfile.py``) on the import path. This would make certain imports work
+  in editable installations but not regular installations.