Use the pub cache resolved by pub itself.
To add packages to the flutter.zip download they are packaged as tar.gz and added to the pub-cache on first run by using `pub cache preload`.
...and various other minor cleanup:
* Moved "FLUTTER_STORAGE_BASE_URL" into a constant throughout the code. There are other strings that we should do that to but this one was relevant to the code I was changing.
* Fixed the logger's handling of slow warnings. Previously it deleted too much text. Fixed the test for that to actually verify it entirely, too.
* Made the logger delete the slow warning when it's finished.
* Fixed 'Please choose one (To quit, press "q/Q")' message to be the cleaner 'Please choose one (or "q" to quit)'.
* Added a debug toString to ValidationResult for debugging purposes (not used).
* In http_host_validator:
- Shortened constant names to be clearer (e.g. kPubDevHttpHost -> kPubDev).
- Added GitHub as a tested host since when you run `flutter` we hit that immediately.
- Renamed the check "Network resources".
- Updated the `slowWarning` of the check to say which hosts are pending.
- Removed all timeout logic. Timeouts violate our style guide.
- Removed `int.parse(... ?? '10')`; passing a constant to `int.parse` is inefficient.
- Replaced the `_HostValidationResult` class with `String?` for simplicity.
- Improved the error messages to be more detailed.
- Removed all checks that dependened on the stringification of exceptions. That's very brittle.
- Added a warning specifically for HandshakeException that talks about the implications (MITM attacks).
- Replaced exception-message-parsing logic with just calling `Uri.tryParse` and validating the result.
- Replaced a lot of list-filtering logic with just a single for loop to check the results.
- Replaced code that added a constant to a known-empty list with just returning a constant list.
- Revamped the logic for deciding which hosts to check to just use a single chain of if/else blocks instead of getters, lists literals with `if` expressions, `??`, functions, etc spread over multiple places in the code.
This modifies the flutter update-packages and flutter update-packages --force-upgrade commands so that the many invocations of "dart pub get" in each repo project run in parallel instead of in series.
