Resolve Warnings about intialization of xcube plugin

[TejasMorbagal]

Describe the bug

Warnings about intialization of xcube plugin

To Reproduce
Steps to reproduce the behavior:

from xcube.core.store import new_data_store
store = new_data_store("ccizarr")

Expected behavior
No ugly warning when new_data_store is called.

Additional context

UserWarning: Initializing xcube plugin 'xcube_cmems' took 116 ms, consider code optimization.
 (For example, avoid eager import of packages, consider lazy loading of resources, etc.)
  warnings.warn(

[forman]

And have you considered code optimization? ;-)

[TejasMorbagal]

No not yet. I can take up this task next week.