cmd/cue: cue mod rename command

[rogpeppe]

It's useful to be able to rename modules, particularly when bootstrapping new modules.
In particular, the module path chosen by cue mod init by default is arbitrary and should
not be used when publishing.

We propose a new command, cue mod rename which will automatically rename
the current module, and also rewrite import directives in source files that import packages from
the current module so that they use the new name.

Example usage test:

exec cue mod rename my.domain/q
cmp cue.mod/module.cue new/cue.mod/module.cue
cmp q.cue new/q.cue
cmp x/x.cue new/x/x.cue
cmp y/y.cue new/y/y.cue

-- cue.mod/module.cue --
module: "cue.example"

-- q.cue --
package q

q: 3
-- x/x.cue --
package x

import "cue.example/y"

x: y.y
-- y/y.cue --
package y

import "cue.example:q"
y: q.q

-- new/cue.mod/module.cue --
module: "my.domain/q"

-- new/q.cue --
package q

q: 3
-- new/x/x.cue --
package x

import "my.domain/q/y"

x: y.y
-- new/y/y.cue --
package y

import "my.domain/q"
y: q.q

For #3620