Construct tarball url for beta versions correctly

https://issues.sonatype.org/browse/NX-634

When converting the tarball url for npm packages from the remote url to the Nexus url, an assumption was made about the tarball filename that was incorrect for beta versions.

For example, the tarball for the handlebars@1.0.5-beta version is supposed to be:
.../handlebars/-/handlebars-1.0.5beta.tgz

but Nexus was constructing it as:
.../handlebars/-/handlebars-1.0.5-beta.tgz