Add an option to relax semantic equivalence requirement on compile package

[tsandall]

Currently the opa/compile package and the opa build command (which is implemented on top of the compile package) guarantees that emitted bundles are semantically equivalent to the input .rego files provided by the caller. This means that any query that could be run against the provided .rego files can also be run against the emitted bundle. This is nice in cases where the bundle is to be fed back into OPA, because callers of OPA can be unaware of any optimizations to the bundle.

On the other hand, in some cases, it's not possible to produce a semantically equivalent bundle because of limitations like the one discussed in #4411. For example:

package x

p[1]

f(x) { p[x] }

Will fail to build w/ entrypoint x/p because the function f refers to the entrypoint (0402892d8ee5a223453ee41cf4bfad446f11aea2):

opa build -t wasm -e x/p foo.rego

In many compiler use cases, the consumer may not require semantically equivalent bundles. In those cases, we should relax the requirement on the compile package. A simple option like --prune-unused on opa build and a similar option on the compile package would be enough.