ast+compile+cmd: Support entrypoint annotations. (#5246)

This commit adds support to the `compile` package for extracting
entrypoints from Rego `entrypoint` metadata annotations. The new
entrypoint annotations can be applied to any valid entrypoint target,
e.g. rules and packages.

An example policy using the new entrypoint annotations:

    package test

    # METADATA
    # entrypoint: true
    allow {
        input.x
    }

The `build` and `eval` CLI commands have been updated so that they do
not require an entrypoint to be explicitly provided with `-e`, so long
as at least one valid rule/package is marked with an entrypoint
annotation.

The new feature is additive; entrypoints can still be explicitly
provided to these commands by `-e`, and those will be used alongside the
entrypoints discovered from the metadata annotations.

Fixes: #3459

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>