Fixes for cran

[prabhu]

• Corrected Typo: The non-standard attribute is_case_sensitve in name_definition has been corrected to the standard case_sensitive.
• Defined Requirements: The name_definition and version_definition are now correctly marked as "required", as they are mandatory for a valid cran purl.
• Improved Name Definition: The note in the name_definition has been updated to be more specific, clarifying that while the name is case-sensitive, the canonical case spelling MUST be used because CRAN enforces case-insensitive uniqueness.
• Added Qualifiers Definition: An empty qualifiers_definition has been added to the schema to explicitly state that the cran purl type does not use any qualifiers.

[maennchen]

Needs to be rebased as of #640