Add package_content PurlDB field on Package model

[tdruez]

Issue: #307

Changes

• If multiple entries are found are found in the PurlDB, leverage the package_content value, when available, to select a "source" package.

New "update from PurlDB" matching logic:

• If exactly one match is found, its data is used directly.
• If multiple entries are found, leverage the package_content value, when available, to select a "source" package.
• If multiple entries are found, only values that are non-empty and common across all entries are merged and used to update the Package.