improve error message when wasm binary is missing package version

[cargopete]

Fixes #117.

When wkg publish encounters a Wasm binary that has no version in the embedded package metadata (because the WIT package statement was written without a @version), the previous error message was:

Error: malformed component: component package version not found

This gives no indication of what went wrong or how to fix it. The improved message explains the cause and offers two remediation paths:

Error: malformed component: component package version not found in the Wasm binary

The Wasm file was built without a version in the WIT `package` statement.
Add a version to the `package` statement in your .wit file, e.g.:

    package example:my-package@1.0.0;

Alternatively, specify the package and version explicitly with the --package flag:

    wkg publish <file> --package <namespace>:<name>@<version>