Split out of #18662: the converter/manifest hardening without the
download parallelization, which stays there.
The PIO extra-script shim moves from espidf/ to platformio/ and gains
type checks for every manifest field, capture of the Append variants and
CPPPATH, warn-once diagnostics for unmodelled env reads, and uniform
shlex quoting so captured values survive the lex_build_flags round-trip.
The dependency walk normalizes every spelling PIO accepts, splits the
routine cross-platform skip (IncompatiblePlatform) from real manifest
problems, validates manifest shape once, and the shared flag lexer
warns and drops trailing or empty glued arguments.
J. Nick Koston
9731c9527a