Add new MavenPackage build system base class (#18185)

* Add new MavenPackage build system base class

* Fix flake8 and doc tests

* More specific regex

* Java 8 required for these packages

lib/spack/docs/build_systems.rst

@@ -29,6 +29,7 @@ on these ideas for each distinct build system that Spack supports:
    :maxdepth: 1
    :caption: Make-incompatible
 
+   build_systems/mavenpackage
    build_systems/sconspackage
    build_systems/wafpackage
 

lib/spack/docs/build_systems/mavenpackage.rst

@@ -0,0 +1,84 @@
+.. Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+   Spack Project Developers. See the top-level COPYRIGHT file for details.
+
+   SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+.. _mavenpackage:
+
+------------
+MavenPackage
+------------
+
+Apache Maven is a general-purpose build system that does not rely
+on Makefiles to build software. It is designed for building and
+managing and Java-based project.
+
+^^^^^^
+Phases
+^^^^^^
+
+The ``MavenPackage`` base class comes with the following phases:
+
+#. ``build`` - compile code and package into a JAR file
+#. ``install`` - copy to installation prefix
+
+By default, these phases run:
+
+.. code-block:: console
+
+   $ mvn package
+   $ install . <prefix>
+
+
+^^^^^^^^^^^^^^^
+Important files
+^^^^^^^^^^^^^^^
+
+Maven packages can be identified by the presence of a ``pom.xml`` file.
+This file lists dependencies and other metadata about the project.
+There may also be configuration files in the ``.mvn`` directory.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^
+Build system dependencies
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Maven requires the ``mvn`` executable to build the project. It also
+requires Java at both build- and run-time. Because of this, the base
+class automatically adds the following dependencies:
+
+.. code-block:: python
+
+   depends_on('java', type=('build', 'run'))
+   depends_on('maven', type='build')
+
+
+In the ``pom.xml`` file, you may see sections like:
+
+.. code-block:: xml
+
+   <requireJavaVersion>
+      <version>[1.7,)</version>
+   </requireJavaVersion>
+   <requireMavenVersion>
+      <version>[3.5.4,)</version>
+   </requireMavenVersion>
+
+
+This specifies the versions of Java and Maven that are required to
+build the package. See
+https://docs.oracle.com/middleware/1212/core/MAVEN/maven_version.htm#MAVEN402
+for a description of this version range syntax. In this case, you
+should add:
+
+.. code-block:: python
+
+   depends_on('java@7:', type='build')
+   depends_on('maven@3.5.4:', type='build')
+
+
+^^^^^^^^^^^^^^^^^^^^^^
+External documentation
+^^^^^^^^^^^^^^^^^^^^^^
+
+For more information on the Maven build system, see:
+https://maven.apache.org/index.html