simplify use of default_tag

[demoray]

Most of the time, tag is analogous to "API version", but not always. The idea is to allow the following:

• Allow the generated crate user to specify which tags they want if they want something specific
• Allow the SDK developer to rely on the "usual" behavior where it's API version, and use the "latest" without having to continuously update the SDK crate based on a steady churn of the generated crates
• When user uses both the generated crate and the SDK-crate, it doesn't violate either of the above ideals
• Allow users to use both generated crates and the SDK-crate without multiple imports of the same crate as the generated crates can be huge. (For reference, azure_svc_blobstorage is ~5M of source, and each feature is ~1M)

This change modifies the generated import behavior thusly:

• Any tag that is selected by feature name is available for use via use crate_name::tag_name::models;
• If the default_tag feature is selected, then the tag is included by feature name and can be used via use crate_name::tag_name::models;
• If the default_tag feature is selected, then the implementation for that specific tag is imported at the top level for the crate, such that it can also be used as use crate_name::models;
• Import behavior no longer changes based on features specified, which was a "footgun" that users hit because rust Crate feature flags are additive