Move the uber `mock` dependency to the `mock` subpackage

[Rueian]

Following the #447,

We should continue to reduce unnecessary dependencies for users.

The go.uber.org/mock is only used in the mock subpackage. It will be better to move the dependency to mock's go.mod.

[Shivam Chaturvedi]

This is also used in rueidishook package.

[Prashant Rewar]

/assign