textlogger: don't leak internal VState in the Config API

[pohly]

/kind bug

What steps did you take and what happened:

Config embeds VState, which makes VState methods V and VModule accessible. This was intentional because it enables verbosity level changes at runtime. However, it's not a good solution because the internal packages should be completely hidden.

What did you expect to happen:

Config should define its own API for v and vmodule changes. This might even be the same as in VState.

Advantages:

• more discoverable
• proper layering

[pohly]

/assign

[logicalhan]

/triage accepted