This change harmonizes j2cl:build and j2cl:watch outputs.

Now that :watch copies resources like index.html to ${webappsDirectory}/${artifactId}/,
it's not clear which <script> src to use for the output js
(:watch needs src="/${artifactId}.js" to resolve, and :build
needs src="/${artifactId}/${artifactId}.js").

By setting :build's initialScriptFilename = ${artifactId}.js by default,
index.html can simply use <script src="/${artifactId}.js"></script> without
confusion.