Make `inject-ecj-compiler.pl` smarter

[wtwhite]

As discussed here, inject-debug-mode-compiler.pl's injections fail to take effect when the POM already contains maven-compiler-plugin <plugin> elements, and inject-ecj-compiler.pl will fail the same way, likely causing part/all of these projects to be compiled with the regular OpenJDK compiler instead of the Eclipse compiler.

I also noticed that there are some <plugin>s that themselves contain a <plugin> element, e.g., from checkstyle-10.12.3's POM:

        <plugin>
          <groupId>com.github.spotbugs</groupId>
          <artifactId>spotbugs-maven-plugin</artifactId>
          <version>${maven.spotbugs.plugin.version}</version>
          <configuration>
            <effort>Max</effort>
            <threshold>Low</threshold>
            <excludeFilterFile>config/spotbugs-exclude.xml</excludeFilterFile>
            <spotbugsXmlOutputDirectory>target/spotbugsreports</spotbugsXmlOutputDirectory>
            <plugins>
              <plugin>
                <groupId>com.mebigfatguy.sb-contrib</groupId>
                <artifactId>sb-contrib</artifactId>
                <version>7.6.0</version>
              </plugin>
            </plugins>
          </configuration>
        </plugin>

This will be causing inject-ecj-compiler.pl to perform incorrect extra injections, possibly breaking things.