addon FsVariables is being built as PEAR, but isn't one

For the 2.3.0 release, this was built by common build strategy in SandboxDistr pom; this put all the code to be assembled into a directory assembly-bin/<artifactId>.

For 2.3.1, the top parent pom for UIMA has a profile intended to handle this case, activated by the existence of a file marker-file-identifying-single-project. Change to use this style.