micronaut-maven-plugin POC

This PR contains a POC that fixes #104:

Converts the project into a multi-module Maven build.
Provides a new io.micronaut.build:micronaut-entrypoint artifact with a single class (io.micronaut.build.entrypoint.Entrypoint) that is meant to be the Main-Class of the runnable jars.
- The class loader used is a URLClassLoader that loads dependencies (included in the JAR's libs/ folder) by copying them to the local file system first, since nested JAR URLs aren't supported by URLClassLoader. Ideally, this should be replaced by a custom class loader that is able to read nested JAR URLs.
Includes a micronaut-entrypoint-test test application, coming from a vanilla mn create-cli-app, with the following changes to its POM:
- Copies dependencies to classes/libs so that they end up being inside the JAR.
- Unpacks micronaut-entrypoint in the classes directory, since it needs to be exploded to be the JAR's main class.
- Configure the generated JAR's manifest:
  - Class-Path entry contains all the dependencies in the format libs/foo-1.2.3.jar.
  - Main-Class is fixed to io.micronaut.build.entrypoint.Entrypoint.
  - Micronaut-Entrypoint points to the actual main class (io.micronaut.build.entrypoint.test.MainCommand in this case).
- Ideally, a MOJO should be provided to perform these tasks, so that the user POMs are cleaner.

The generated JAR looks like:

.
├── META-INF
│   ├── MANIFEST.MF
│   ├── maven
│   │   └── ...
│   └── native-image
│       └── ...
├── application.yml
├── io
│   └── micronaut
│       └── build
│           └── entrypoint
│               ├── Entrypoint.class        // Unpacked
│               └── test
│                   └── MainCommand.class.  // The actual main class
├── libs
│   ├── apiguardian-api-1.1.0.jar
│   ├── jackson-annotations-2.11.2.jar
│   ├── jackson-core-2.11.2.jar
│   ├── jackson-databind-2.11.2.jar
│   ├── jackson-datatype-jdk8-2.11.2.jar
│   ├── jackson-datatype-jsr310-2.11.2.jar
│   ├── javax.annotation-api-1.3.2.jar
│   ├── javax.inject-1.jar
│   ├── jsr305-3.0.2.jar
│   ├── junit-jupiter-api-5.7.0.jar
│   ├── junit-jupiter-engine-5.7.0.jar
│   ├── junit-platform-commons-1.7.0.jar
│   ├── junit-platform-engine-1.7.0.jar
│   ├── logback-classic-1.2.3.jar
│   ├── logback-core-1.2.3.jar
│   ├── micronaut-aop-2.3.3.jar
│   ├── micronaut-buffer-netty-2.3.3.jar
│   ├── micronaut-core-2.3.3.jar
│   ├── micronaut-http-2.3.3.jar
│   ├── micronaut-http-client-2.3.3.jar
│   ├── micronaut-http-client-core-2.3.3.jar
│   ├── micronaut-http-netty-2.3.3.jar
│   ├── micronaut-inject-2.3.3.jar
│   ├── micronaut-picocli-3.2.0.jar
│   ├── micronaut-runtime-2.3.3.jar
│   ├── micronaut-test-core-2.3.2.jar
│   ├── micronaut-test-junit5-2.3.2.jar
│   ├── micronaut-validation-2.3.3.jar
│   ├── micronaut-websocket-2.3.3.jar
│   ├── netty-buffer-4.1.59.Final.jar
│   ├── netty-codec-4.1.59.Final.jar
│   ├── netty-codec-http-4.1.59.Final.jar
│   ├── netty-codec-http2-4.1.59.Final.jar
│   ├── netty-codec-socks-4.1.59.Final.jar
│   ├── netty-common-4.1.59.Final.jar
│   ├── netty-handler-4.1.59.Final.jar
│   ├── netty-handler-proxy-4.1.59.Final.jar
│   ├── netty-resolver-4.1.59.Final.jar
│   ├── netty-transport-4.1.59.Final.jar
│   ├── opentest4j-1.2.0.jar
│   ├── picocli-4.6.1.jar
│   ├── reactive-streams-1.0.3.jar
│   ├── rxjava-2.2.10.jar
│   ├── slf4j-api-1.7.26.jar
│   ├── snakeyaml-1.26.jar
│   ├── spotbugs-annotations-4.0.3.jar
│   └── validation-api-2.0.1.Final.jar
└── logback.xml

Mar 01 '21 12:03 alvarosanchez

Spring's "repackage" goal (in the Spring Boot Maven Plugin), does something similar to this, but as you mentioned above, removes the need for unpack the jar first. https://github.com/spring-projects/spring-boot/tree/v2.5.2/spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader

The actual "spring" dependencies on that project are minimal (if any at all, the last I looked)

The resulting package ends up looking like this:

.
├── META-INF
│   ├── MANIFEST.MF
│   ├── ...
│ 
├── BOOT-INF
│   ├── classes
│   │   ├── META-INF
│   │   │   └── rest-dice-parser.kotlin_module
│   │   ├── application.properties
│   │   └── com
│   │       └── example
│   │           └── app
│   │               ├── MyCode.class
│   │               ├── ...
│   └── lib
│       ├── my-deps-1.2.3.jar
│       ├── ...
|
└── org
    └── springframework
        └── boot
            └── loader
                ├── ClassloaderRelatedBits.class
                ├── ...

I've been using this even on non-Spring projects just to avoid using the Shade plugin.