NEXUS-2450: Maven1 reposes will be asked for EJBs

Nexus doing the "conversion" from M2 to M1 path does not have
all the knowledge about requested artifact, like it's packaging
(it's just been asked for). This change adds a small "twist" to
Nexus layout converting shadows, and the "ejbs" folder will be asked
too before giving up a request. While methods now
return List of paths (instead of just one path as before), nothing
changes and almost every method except the doRetrieveItem will
make use only of the first element of the returned list, yielding
in no change. The doRetrieveItem method will use "extra" paths,
and will iterate of the list of paths trying to get response,
ultimately failing with INFex.
NEXUS-2450: NPE fix

Initial implementation invoked parts it should not invoke
even with null result, causing NPE.

Problem was badly located logic within the for-loop.
Switched from direct ArrayList use to guava Lists
Merge pull request #813 from sonatype/nexus-2450-m1-ejbs

[REVIEW] NEXUS-2450: [M1 to M2] EJB artifacts are served from **/jars/ directory instead of **/ejbs/