modules: ability to use local $CUE_CACHE_DIR/mod/download as a virtual registry

[myitcv]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.13.0-alpha.1.0.20250324111431-1eccc4649d73

go version go1.24.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG gotestjsonbuildtext=1,multipathtcp=0,randseednop=0,rsa1024min=0,tlsmlkem=0,x509rsacrt=0,x509usepolicies=0
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
         GOARM64 v8.0
             vcs git
    vcs.revision 1eccc4649d732a422003716b51b130bcd134a938
        vcs.time 2025-03-24T11:14:31Z
    vcs.modified false
cue.lang.version v0.13.0

Is your feature request related to a problem? Please describe.

Go has the ability to set:

GOPROXY=file://$(go env GOMODCACHE)/cache/download

in order that all module-related commands use the local filesystem as a virtual proxy. i.e. no network access. Even queries like @latest work against the local filesystem virtual proxy.

Describe the solution you'd like

A similar feature in CUE. Whether via CUE_REGISTRY or otherwise.

Describe alternatives you've considered

Updating cue mod registry to have a flag that uses the effective CUE_CACHE_DIR as a read-only source of modules.

Additional context

n/a

[jpluscplusm]

Related to #3403, but not a duplicate.