feat: improve fallback detection checks

[rdeltour]

This PR updates fallback-related checks and refactors the code used
for checking references to publication resources.

The following checks are introduced or updated:

• RSC-032 (new): reports foreign resources with no fallback used in
  content documents. Replaces MED-001, MED-002, and CSS-010
• MED-003 now reports when an img element child of a picture element
  is not a core media type.
• MED-001 is suppressed. It was used to report a video poster attribute
  did not reference a core image media type. It is now reported as RSC-032.
• MED-002 is suppressed. It was used to report HTML elements referencing
  foreign resources without fallback. It is now reported as RSC-032.
• CSS-010 is suppressed. It was used to report references to foreign
  stylesheets with no fallback. It is now reported as RSC-032.
• OPF_040 is now reported when the fallback attribute of a package
  document item element does not points to an existing ID. This was
  previously implemented as a Schematron check (RSC-005) in EPUB 3.x.
• OPF-013 is now a warning. It is reported when a MIME type declared
  inline in content (for instance with an HTML type attribute) does
  not match the MIME type declared in the package document.

The code is refactored as follows:

• the reference/resource registry functionality of the XRefChecker
  class is extracted to new top-level classes in the (new) package
  org.w3c.epubcheck.references.
  • Resource represents a publication resource
  • ResourceRegistry is a registry of Resource instances
  • Reference represents a reference (URL) used anywhere in content
  • ReferenceRegistry is a registry of Reference instances
  • XRefChecker is renamed to ResourceReferencesChecker.
  • ValidationContext contains optional references to ResourceRegistry
    and ReferenceRegistry.
  • fallback chain resolution is now done in a new FallbackChainResolver
    class, when building OPFItem instances from the builders created
    when parsing the package document.
  • XMLHandler has convenience methods used to register references
    to the ReferenceRegistry.

Fix #1304, Fix #1298.