Allow specifying policy in the top-level module line

[AkihiroSuda]

e.g.,

// gomodjail:confined
module example.com/foo

go 1.23

require (
        example.com/mod100 v1.2.3
        example.com/mod101 v1.2.3 // gomodjail:unconfined
        example.com/mod102 v1.2.3
        // gomodjail:unconfined
        example.com/mod103 v1.2.3
)

require (
        // gomodjail:unconfined
        example.com/mod200 v1.2.3 // indirect
        example.com/mod201 v1.2.3 // indirect
)

// policy cannot be specified here because the parser ignores
// the comment lines here
require (
)

This makes the following modules confined: mod100, mod102, and mod201.