Dogfood pmd plugin

[blindpirate]

Context

It's not convincing if p3c project doesn't dogfood its own pmd plugin. This PR applies pmd plugin and rules to p3c-pmd project and fixes two violations.

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[blindpirate]

I can also set up a CI job (Travis/CircleCI/Appveyor, etc.) if necessary.

[blindpirate]

Not sure if this is good, please advise.

[xuantan]

你好，目前我们的插件是从JDK1.7版本开始支持。

[blindpirate]

好的。根据https://maven.apache.org/plugins/maven-pmd-plugin/examples/targetJdk.html 的建议，将这个值改为：${maven.compiler.target}

[kerie]

还需要在<properties>里加上maven.compiler.target这个属性吧？

<maven.compiler.source>1.7</maven.compiler.source>
<maven.compiler.target>1.7</maven.compiler.target>

下面maven-compiler-plugin里也统一改成这种形式。

[SeanCai]

@blindpirate

你好，插件最新版已经更新
pmd-> 6.15.0
jdk 最低支持 1.8
另外 p3c-pmd 加入了 p3c规约检测

你那边帮忙看看是否有需要改进的，因为改动较大，所以你本次pr可能不再适用了，十分抱歉

[XenoAmess]

suggest do NEVER do clinic dependency with a specific version.
use it with a range instead.

                    <dependency>
                        <groupId>com.alibaba.p3c</groupId>
                        <artifactId>p3c-pmd</artifactId>
                        <version>[1,)</version>
                    </dependency>

or even:

                    <dependency>
                        <groupId>${project.groupId}</groupId>
                        <artifactId>${project.artifactId}</artifactId>
                        <version>[1,${project.version}]</version>
                    </dependency>