When `nvm use` is given an exact `vX.Y.Z` that is already installed,
and no alias with that exact name exists,
use it directly instead of re-resolving it through
`nvm_match_version` and `nvm_ensure_version_installed`,
both of which call `nvm_version`.
`nvm_auto` takes this path on every shell startup with an already-resolved version,
so this removes ~95ms from each new shell
(~330ms -> ~235ms in a clean environment on macOS/zsh).
Fixes#3930
