breaking(compute/pack): use package name from manifest

[Integralist]

The fastly compute deploy command expects the package file to be named the same as the name attribute in the fastly.toml manifest. The fastly compute pack command does not do this, it instead names the package file package.tar.gz and consequently this causes the deploy command to fail.

NOTE: It was probably assumed that if a user was using compute pack they might not be using the CLI for developing an application, so we shouldn't expect them to have a fastly.toml manifest present. In this PR we presume a manifest is present by default and fallback to using package.tar.gz otherwise.