mod: implement replace directive

[rogpeppe]

Go's module system provides a replace directive to allow replacing
one module by another module or by a module that is present in a local
directory.

CUE's modules should implement that too.

[morlay]

When replace directive supported.

We could maintain cue modules in one monorepo.

code layout:

-- a/cue.mod/module.cue --
module: "ghcr.io/morlay/a@v0"
language: {
        version: "v0.8.0"
}
-- a/x.cue --
package a

#Version: "v0.0.0"
-- b/cue.mod/module.cue --
module: "ghcr.io/morlay/b@v0"
language: {
        version: "v0.8.0"
}
deps: {
        "ghcr.io/morlay/a@v0": {
                "v":       "v0.0.0"
                "replace": "../a"
        }
}
-- b/x.cue --
package b

import (
        "ghcr.io/morlay/a"
)

deps: {
        a: "a.#Version"
}
-- c/cue.mod/module.cue --
module: "ghcr.io/morlay/c@v0"
language: {
        version: "v0.8.0"
}
deps: {
        "ghcr.io/morlay/a@v0": {
                "v":       "v0.0.0"
                "replace": "../a"
        }
}
-- c/x.cue --
package c

import (
        "ghcr.io/morlay/a"
)

deps: {
        a: "a.#Version"
}

module deps:

graph TD
a["ghcr.io/morlay/a"]
b["ghcr.io/morlay/b"]
c["ghcr.io/morlay/c"]

a --> b;
a --> c;

Loading

Now we start to publish all cue modules.

First

cue publish ghcr.io/morlay/a v0.1.0

Then

cue publish ghcr.io/morlay/b v0.1.0
cue publish ghcr.io/morlay/c v0.1.0

but version of ghcr.io/morlay/a already v0.1.0,
we have to change the version of ghcr.io/morlay/a in ghcr.io/morlay/b and ghcr.io/morlay/c, before publish.

So. may consider added optional version?: string in module.cue,
Thencue publish ./a could use the module and version form module.cue
And in the case above, cue publish ./a and cue publish ./c could resolve the correct version of ghcr.io/morlay/a.