Vendored pnpm deployments #957

Vendored pnpm deployments run pnpm install instead of pnpm rebuild

Summary

Since v1.9.5 pnpm is officially supported (big thanks) but pnpm projects with a pre-populated node_modules/ (vendored deployments) incorrectly run pnpm install --no-frozen-lockfile instead of pnpm rebuild.
There are two distinct bugs in BuildDependencies().

Bug 1 — case s.UsePNPM has priority over case s.IsVendored

supply.go#L342–L363

switch {
case s.UseYarn:     // L343
    s.Yarn.Build(...)
case s.UsePNPM:     // L348 ← matched first, PNPM.Build() runs
    s.PNPM.Build(...)
case s.IsVendored:  // L353 ← never reached when pnpm-lock.yaml is present
    s.NPM.Rebuild(...)
...
}

When both flags are set:

• s.UsePNPM = true — pnpm-lock.yaml exists
• s.IsVendored = true — node_modules/ exists

PNPM.Build() runs pnpm install, ignoring the pre-populated node_modules/.

Bug 2 — case s.IsVendored hardcodes NPM.Rebuild() regardless of package manager

Even if the ordering were fixed, case s.IsVendored at
supply.go#L353
always calls s.NPM.Rebuild(). A vendored pnpm project should call PNPM.Rebuild(),
which would also require adding Rebuild(string) error to the
PNPM interface at L51–L53
(today only Build(string, string) error exists),
mirroring npm/npm.go#L34–L51.

The same gap likely exists for Yarn vendored deployments.

Use case

We use pnpm deploy --prod to produce a flat node_modules/ layout compatible with the CF buildpack, then push the archive.
The buildpack should detect the pre-populated node_modules/ and skip the install phase, running pnpm rebuild instead to recompile any native addons for the CF stack.

How to reproduce

1. Create a minimal pnpm workspace with a pnpm-lock.yaml and a pre-populated node_modules/.
2. Run the buildpack supply step.
3. Expected: pnpm rebuild is called.
4. Actual: pnpm install --no-frozen-lockfile is called.