Avoid making purl2vcs calls in the index_packages view function

[keshav-space]

• index_packages is meant for bulk indexing of packages asynchronously. Since purl2vcs involves network call, it should not be called in the view function. Instead, purl2vcs should be called while processing the PriorityResourceURI.
• Also, currently purl2vcs is only called when reindexing is requested. It should also be called when indexing new packages.