Validate PURL syntax

[DennisClark]

Extend syntax validation for PURL

Deliverable: Update and release FetchCode library to infer PURL download URLs for the remaining ecosystems, identified by PURL-TYPES, that need to be supported as needed by the community, based on the spec types: https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst

Provide more comprehensive support for PURL validation, considering the differences that occur when certain values appear in the PURL components.