The wasmer package tag command should not need a manifest

[xdoardo]

Right now the wasmer package tag command requires the package's manifest to be specified in some way, either explicitly giving its path or having it in the current working directory.

We should use the manifest only as a fallback in case the user did not provide every information necessary: as an idea, we could have the command in the format

wasmer package tag sha256:abcd...... namespace/name --version 0.1.0

with the second positional argument (namespace/name) being optional and filled in from the manifest if present, and the same for the version.