[Perf] nvm_resolve_local_alias: skip nvm_version for an exact, installed version

[hridhinvarkeyjoseph]

Part of #3416 (the second cost described in #3416 (comment)).

When an alias resolves to an exact vX.Y.Z that is already installed, nvm_resolve_local_alias still passes it through nvm_version, which can't change the result. This returns it directly after an nvm_is_version_installed check. Anything else (partial versions, node, lts/*, iojs, not-installed versions) still goes through nvm_version as before.

In the fast test fixtures this took nvm_resolve_local_alias on an exact alias from ~63ms to ~22ms per call.

This does not address the bigger cost in the original report, where default -> node has to resolve the node/stable implicit aliases. Happy to look at that separately, or to fold this into #3937 if you'd prefer.

Adds a test that mocks nvm_version to confirm it's skipped for an exact installed version and still used for one that isn't installed.

[hridhinvarkeyjoseph]

@ljharb I've also cherry-picked this commit onto #3937, since both are the same exact-version fast path on the startup route. Would you prefer to review them together there (and I'll close this one), or keep this as a separate PR?

The 3 failing checks look unrelated to the change: the two WSL nvm install jobs timed out after 45m, and xenial (bash) failed after 1m. Happy to look into any of them if they're not known flakes.

[hridhinvarkeyjoseph]

Closing in favour of #3937, which now includes this commit, so there's one PR for this startup fast path instead of two overlapping ones.

[ljharb]

In the future, please don't ever close a PR you've opened; PR refs are permanent and can never be removed.

[hridhinvarkeyjoseph]

@ljharb Understood, sorry about that. I've reopened this.

Since #3937 also has this commit cherry-picked, which would you prefer?

• Review the two separately, and I'll drop the commit from [Perf] nvm use: skip re-resolving an exact, installed version #3937.
• Keep it in [Perf] nvm use: skip re-resolving an exact, installed version #3937, and close this one whenever it suits you.

[hridhinvarkeyjoseph]

Folded into #3937.

[hridhinvarkeyjoseph]

Sorry, I closed this by mistake despite your earlier note — reopened. Its commit is already in #3937 (now approved, with your tweak), so please close this one whenever it suits you.