Set src to lib.cleanSource projectDir by default

Ideally we'd use `nix-gitignore` but that one fails when there is no
`.gitignore` file so it's not a sensible default.

README.md

@@ -18,8 +18,8 @@ The _poetry2nix_ public API consists of the following attributes:
 
 Creates a Python application using the Python interpreter specified based on the designated poetry project and lock files. `mkPoetryApplication` takes an attribute set with the following attributes (attributes without default are mandatory):
 
-- **src**: project source.
 - **projectDir**: path to the root of the project.
+- **src**: project source (_default_: `lib.cleanSource projectDir`).
 - **pyproject**: path to `pyproject.toml` (_default_: `projectDir + "/pyproject.toml"`).
 - **poetrylock**: `poetry.lock` file path (_default_: `projectDir + "/poetry.lock"`).
 - **overrides**: Python overrides to apply (_default_: `[defaultPoetryOverrides]`).
@@ -29,7 +29,6 @@ Creates a Python application using the Python interpreter specified based on the
 #### Example
 ```nix
 poetry2nix.mkPoetryApplication {
-    src = lib.cleanSource ./.;
     projectDir = ./.;
 }
 ```

default.nix

@@ -145,8 +145,8 @@ let
 
   /* Creates a Python application from pyproject.toml and poetry.lock */
   mkPoetryApplication =
-    { src
-    , projectDir ? null
+    { projectDir ? null
+    , src ? lib.cleanSource projectDir
     , pyproject ? projectDir + "/pyproject.toml"
     , poetrylock ? projectDir + "/poetry.lock"
     , overrides ? [ defaultPoetryOverrides ]
@@ -199,6 +199,8 @@ let
           pname = pyProject.tool.poetry.name;
           version = pyProject.tool.poetry.version;
 
+          inherit src;
+
           format = "pyproject";
 
           buildInputs = mkInput "buildInputs" buildSystemPkgs;

tests/trivial/default.nix

@@ -2,7 +2,5 @@
 
 poetry2nix.mkPoetryApplication {
   python = python3;
-  pyproject = ./pyproject.toml;
-  poetrylock = ./poetry.lock;
-  src = lib.cleanSource ./.;
+  projectDir = ./.;
 }