Normalize and document the format for referring to templates

[da-tanabe]

Since the introduction of package IDs in DAML many years ago, a few unofficial and ad-hoc stringifications of templates have existed, and dazl has attempted to support them all, and has additionally never had an official one.

Standardize on the current format as defined by https://docs.daml.com/json-api/index.html:

• <package ID>:<module>:<template>
• <module>:<template>

Older formats should continue to work, but a deprecation warning will be printed if they are used:

• <module>.<template>@<package ID>
• <module>:<template>@<package ID>
• <module>.<template>

Ensure that this format is documented as well.