Prepend mojo shebang to skyx files

skyx files are zips, so they can have anything at the start. Having
a shebang line at the start makes it easier to run skyx files in a mojo
environment.

With this patch, we can switch over to using this command from the build system
in the sky_engine repo.

This command will produce an flx package. Currently, this command doesn't work
because we don't have the Flutter compiler downloaded from Google storage yet.
A future patch will make that happen.

This stray line was causing an analyzer error.

Add very-verbose flag and automatically log some more process-related things in verbose and very-verbose modes.

Also applies autoreformatting to init.dart.

This command uses package:test to run Dart tests with sky_shell.  For this to
work, we need https://github.com/dart-lang/test/tree/hacky-loader-hook to land.
We're also not smart enough to find sky_shell ourselves yet. Instead, we take
the path as input using an environment variable. Eventually, we'll be able to
get the sky_shell executable from package:sky_engine, but we don't yet ship
that executable.

This is needed to make pub run sky_tools:sky_server work.
Partial fix for https://github.com/domokit/sky_engine/issues/539

TBR=abarth

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