[422147] Unnecessary call to the SourceModuleInfoCache

Optimize the code of
SourceParserUtil.getModuleDeclaration(ISourceModule) to avoid
unnecessary calls to SourceModuleInfoCache.get(ISourceModule). 

This method is synchronized and any unnecessary calls to it increase the
chance for bottlenecks if multiple threads are trying to traverse AST
trees.

Bug: 422147
Change-Id: I1427e3376aca67fd7dd95b6c4744c6f9d8261c6a
Signed-off-by: Kaloyan Raev <kaloyan.r@zend.com>