use ${maven.multiModuleProjectDirectory} to locate plugins dir

<plugin.dest.dir>${maven.multiModuleProjectDirectory}/skywalking-agent/plugins</plugin.dest.dir>

replace previously defined properties

<sdk.plugin.related.dir />
<agent.package.dest.dir>${project.build.directory}${sdk.plugin.related.dir}/../../../../skywalking-agent
</agent.package.dest.dir>
<plugin.dest.dir>${agent.package.dest.dir}/plugins</plugin.dest.dir>

especially，we can avoid the annoying sdk.plugin.related.dir property

the same to bootstrap.plugin.dest.dir expired.plugin.dest.dir optional.plugin.dest.dir optional.reporter.plugins.dest.dir

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes #.
• Update the CHANGES log.