pkginfo.install_dir() returns nil for a dependency that exists only in the shared registry cache

[wellwei]

Problem

pkginfo.install_dir(ns:name, ver) returns nil for a dependency that is installed on disk — in the shared registry cache ($MCPP_HOME/registry/data/xpkgs/<ns>-x-<name>/<ver>) — but is not present in the installing package's member-local xpkgs.

The fallback scan _resolve_dep_via_scan only searches:

1. _RUNTIME.xpkg_dir
2. path.directory(path.directory(_RUNTIME.install_dir)) (member-local xpkgs)
3. _RUNTIME.project_data_dir/xpkgs

It never searches the shared cache. So a dependency that was installed there (e.g. built by a sibling workspace member and reused by this member) is invisible to install hooks.

Minimal repro (mcpp 2026.8.8.2, macOS arm64)

Same workspace as above, but install() records what install_dir() says about a package seeded into the shared cache:

-- pkgs/r/repro-hook.lua — mcpp.deps declares zlib (NOT xpm.deps), mirroring the
-- original connector descriptor.
mcpp = {
    sources = { "repro_anchor.c" },
    targets = { ["repro_hook"] = { kind = "lib" } },
    deps = { ["compat.zlib"] = "1.3.2" },
}
function install()
    local z = pkginfo.install_dir("compat:zlib", "1.3.2")
    -- append "install_dir(compat:zlib@1.3.2) = <z>" to a result file
    return false
end

Steps:

1. Seed the shared cache: cp -R <an existing compat-x-zlib install> $MCPP_HOME/registry/data/xpkgs/compat-x-zlib/1.3.2
2. mcpp test -p tests/examples/repro-hook (fresh member dir; zlib not member-local)

Result file:

install_dir(compat:zlib@1.3.2)  = nil
shared cache has zlib         = true      -- zlib IS on disk in the shared cache

Control (no seed — zlib installs member-local):

install_dir(compat:zlib@1.3.2)  = .../tests/examples/repro-hook/.mcpp/.xlings/data/xpkgs/compat-x-zlib/1.3.2
shared cache has zlib         = false

Real-world impact

mcpplibs/mcpp-index#185: compat.mysql-connector-cpp's install hook called pkginfo.install_dir("compat:openssl","3.5.1"); openssl was built into the shared cache by the sibling libmysqlclient member and reused, so the hook got nil and bailed with the empty E_INTERNAL (see #513). The descriptor had to add its own fallback that scans $MCPP_HOME/registry/data/xpkgs.

Related: #484 / #485 (recording the resolved dependency totally would also fix this path — when a resolver record with install_dir exists, install_dir() works; it is the scan fallback / missing record that misses the shared cache).

Expected

install_dir() should locate a dependency that is present in the shared registry cache — either by extending _resolve_dep_via_scan to include $MCPP_HOME/registry/data/xpkgs, or by ensuring the resolver records install_dir for every resolved dependency (including ones reused from the shared cache).