Coloquei as seguinte dependência que o professor disponibiliza no curso, porem ela não importa a lib do json.

<dependency>
    <groupId>javax</groupId>
    <artifactId>javaee-api</artifactId>
    <version>7.0</version>
    <scope>provided</scope>
</dependency>

Meu pom

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>br.com.casadocodigo</groupId>
    <artifactId>casadocodigo</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <packaging>war</packaging>
    <build>
        <finalName>casadocodigo</finalName>
    </build>
    <properties>
        <failOnMissingWebXml>false</failOnMissingWebXml>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.jboss.spec.javax.faces</groupId>
            <artifactId>jboss-jsf-api_2.1_spec</artifactId>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>javax.enterprise</groupId>
            <artifactId>cdi-api</artifactId>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>javax.annotation</groupId>
            <artifactId>jsr250-api</artifactId>
            <scope>provided</scope>
        </dependency>
    </dependencies>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.jboss.spec</groupId>
                <artifactId>jboss-javaee-6.0</artifactId>
                <version>3.0.3.Final</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
            <dependency>
                <groupId>javax.annotation</groupId>
                <artifactId>jsr250-api</artifactId>
                <version>1.0</version>
                <scope>provided</scope>
            </dependency>
            <dependency>
                <groupId>javax</groupId>
                <artifactId>javaee-api</artifactId>
                <version>7.0</version>
                <scope>provided</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
</project>

Código da minha classe que não importa o Json

package br.com.casadocodigo.loja.models;

import java.io.Serializable;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

import javax.enterprise.context.SessionScoped;
import javax.inject.Inject;
import javax.inject.Named;

import br.com.casadocodigo.loja.daos.CompraDao;

@Named
@SessionScoped
public class CarrinhoCompras implements Serializable{
    private static final long serialVersionUID = 1L;

    @Inject
    private CompraDao compraDao;

    private Set<CarrinhoItem> itens =
             new HashSet<>();

     public void add(CarrinhoItem item) {
         itens.add(item);
     }

    public List<CarrinhoItem> getItens() {
        return new ArrayList<CarrinhoItem>(itens);
    }

    public BigDecimal getTotal(CarrinhoItem item) {
        return item.getLivro().getPreco().multiply(
                new BigDecimal(item.getQuantidade()));
    }

    public BigDecimal getTotal() {
        BigDecimal total = BigDecimal.ZERO;
        for (CarrinhoItem carrinhoItem : itens) {
            total = total.add(carrinhoItem.getLivro().getPreco()
                    .multiply(new BigDecimal(carrinhoItem.getQuantidade())));
        }
        return total;
    }

    public void remover(CarrinhoItem item) {
        this.itens.remove(item);
    }

    public Integer getQuantidadeTotal() {
        return itens.stream().mapToInt(item -> item.getQuantidade()).sum();
    }

    public void finalizar(Usuario usuario) {
        Compra compra = new Compra();
        compra.setUsuario(usuario);
        compra.setItens(this.toJson());
        compraDao.salvar(compra);
    }

    private String toJson() {
        Json.createArrayBuilder();
        return "{}";
    }
}