Hello,
Unfortunately Guvnor 5.5 (and prior) only uses that file when (1) building the package and creating the PKG file, and (2) when validating a rule.
More unfortunately (for you) there is no direct means to retrieve the drools.package file either with REST (that I assume you're using for the BRL rule).
You could retrieve the whole package source (IIRC something like /package/{packageName}/source) and parse it for imports, but it's a long winded way especially if the package is large.
That said it's only a one-off need; once you've parsed the imports you can keep them locally.
With kind regards,
Mike