Factor PlatformIO buildgen out of writer.py (#9378)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

esphome/__main__.py

@@ -34,6 +34,7 @@ from esphome.const import (
     CONF_PORT,
     CONF_SUBSTITUTIONS,
     CONF_TOPIC,
+    ENV_NOGITIGNORE,
     PLATFORM_ESP32,
     PLATFORM_ESP8266,
     PLATFORM_RP2040,
@@ -209,6 +210,9 @@ def wrap_to_code(name, comp):
 
 
 def write_cpp(config):
+    if not get_bool_env(ENV_NOGITIGNORE):
+        writer.write_gitignore()
+
     generate_cpp_contents(config)
     return write_cpp_file()
 
@@ -225,10 +229,13 @@ def generate_cpp_contents(config):
 
 
 def write_cpp_file():
-    writer.write_platformio_project()
-
     code_s = indent(CORE.cpp_main_section)
     writer.write_cpp(code_s)
+
+    from esphome.build_gen import platformio
+
+    platformio.write_project()
+
     return 0