Fixes #6491

Only download the minimum*** set of artifacts needed for a particular host, target, and build mode.

*** I still include gradle and fonts in the minimum set.

Adds additional arguments to several Cache methods:

• buildMode & targetPlatform for filtering by artifacts, can be null. If null, retrieve all artifacts matching one of the other parameters. For example, targetPlatform.ios and null buildMode downloads debug, profile, and release for iOS.
  -skipUnknown, which allows downloading artifacts when targetPlatform and hostPlatform are unknown.
• clobber, whether to force re-downloading of artifacts

flutter precache will still download the full set of artifacts for a given host platform.
flutter precache -f/--force will re download all artifacts.

Re-organizes engine artifacts into a class from lists of lists of strings.

Flutter commands now check the artifacts for the given or default build mode, and any connected devices. (note: android-arm-profile is downloaded unconditionally because we currently need it for a Flutter doctor check).

This is accomplished via a new method on FlutterCommand called updateCache. This is only called if shouldUpdateCache is true. The default implementation is:

  Future<void> updateCache() async {
    // Only download the minimum set of binaries.
    await cache.updateAll(
      clobber: false,
      skipUnknown: true,
    );
  }

Run, for example, looks at the current set of attached devices and ensures the binaries are downloaded for each kind.

Example doctor command:

jonahwilliams@jonahwilliams0:~/Documents/flutter/examples/hello_world$ rm -rf ../../bin/cache/
jonahwilliams@jonahwilliams0:~/Documents/flutter/examples/hello_world$ flutter doctor
Downloading Dart SDK from Flutter engine 15f2b92cce916982b7dd8ce658bbf2a465c06ba4...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 50.4M  100 50.4M    0     0  43.4M      0  0:00:01  0:00:01 --:--:-- 43.4M
Building flutter tool...
Downloading Material fonts...                                       1.7s
Downloading package sky_engine...                                   0.3s
Downloading common tools...                                         2.0s
Downloading linux-x64 tools...                                      1.0s
Downloading android-arm-profile/linux-x64 tools...                  1.7s
Downloading Gradle Wrapper...                                       1.0s
Doctor summary (to see all details, run flutter doctor -v):