* [path_provider] Switch to Pigeon for macOS

Converts from direct method channel use to the new experimental Swift
generator. Also switches from a one-call-per-method approach to a single
method with an enum, since it's easy to do that in a maintainable way
with Pigeon (unlike manual method channels, where keeping enum indexes
in sync across the language boundary is error-prone, and requires manual
int/enum conversion).

* Format

* Analyzer

* Update to latest version of Pigeon