Swap `/pom.xml` and `/log4j-bom/pom.xml`

- `/pom.xml` is moved to `/log4j-parent/pom.xml`
- `/log4j-bom/pom.xml` is moved to `/pom.xml`
- Implements the BOM organization described by Maven[1].
  That is, `parent` inherits from `bom`.
- Takes advantage of `flatten-bom` provided by `logging-parent`
- Identical scheme to the one found in `-tools` and `-transformation`

[1] https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#bill-of-materials-bom-poms

log4j-flume-ng/pom.xml

diff --git a/log4j-flume-ng/pom.xml b/log4j-flume-ng/pom.xml
index 9a78d6a..2921027 100644
--- a/log4j-flume-ng/pom.xml
+++ b/log4j-flume-ng/pom.xml
@@ -21,6 +21,7 @@
     <groupId>org.apache.logging.log4j</groupId>
     <artifactId>log4j</artifactId>
     <version>${revision}</version>
+    <relativePath>../log4j-parent</relativePath>
   </parent>
   <artifactId>log4j-flume-ng</artifactId>
   <packaging>jar</packaging>