Support SDK-specific configs (home-path, environment).

[isoos]

• Better environment handling for SDKs, removing almost-duplicate method. #1320 added config home, but there is still a chance that SDKs sharing the same home config directory would override each other's settings. The new SdkConfig ensures that complete separation is possible.
• Also fixing a possible dartdoc glitch (not necessarily bug, rather a hard to debug issue) by explicitly calling the flutter binary (or its embedded dart binary) for Flutter packages. Apparently if FLUTTER_ROOT is set, and we are calling an unrelated Dart SDK, dartdoc implicitly sets the --sdk-dir parameter to <sdk-root>/bin/cache/dart-sdk/, which is absent is the regular Dart SDK, and dartdoc fails.