Related issue #108606.

Nowadays we have multiple outdated versions of Flutter in nixpkgs.
Instead, let's focus in having in having stable versions of Flutter
working.

Users needing to use beta or dev versions of Flutter can use mkFlutter
function, that still exists.

For those that need other versions of Dart, they can use the new
sources parameter instead.

Needs to do a build in 2-pass now since Google stopped shipping all
dependencies needed to build Flutter. This may be an oversight from them
since they used to ship everything, but this makes the whole build
process more robust.

The first step will download all dependencies from pub, and the
second step will build Flutter. Since we need to build repository
first, we also require a new depsSha256 parameter to be passed, that
represents the SHA256 of the resulting derivation of all Flutter
dependencies downloaded from https://pub.dev.

This commit also makes some changes in mkFlutter, allowing the
user to pass src instead of passing version/channel/filename, allowing
for more flexibility (i.e: building from a local fork of Flutter).

…stable-versions