Rename workflowagents package to workflowagent

[rakyll]

ADK will introduce a workflowagent in the future, its code can easily fit into the workflowagent. Plural package names is an anti pattern in Go because the package name is a part of the identifier in code. Consider the difference between before and after:

Before:

workflowagents.New(...)

After:

workflowagent.New(...)

Rename workflowagents to workflowagent to avoid breaking changes in the future.