* Load optional modules dropped into src/modules/optional/
bin/optional-modules.py scans src/modules/optional/ for a directory <Name>/ holding <Name>.h and generates $BUILD_DIR/OptionalModules.h with an include and a setup<Name>() call for each, which Modules.cpp picks up through __has_include. The directory does not exist in a stock checkout, so a stock build generates a header that defines nothing, OPTIONAL_MODULES_SETUP compiles away, and nothing is registered. Sources under the directory are already covered by the default recursive build_src_filter, so dropping a module in needs no platformio.ini edit.
* Address review: skip a module directory that is not a usable identifier
The directory name becomes a setup<Name>() call, so foo-bar/ would have generated setupfoo-bar() and failed to compile with the error pointing at generated code rather than at the directory. Names that cannot form an identifier are now skipped with a message that names the directory.
