Fix doubled warnings being printed for manifest warnings when loading ports.

[BillyONeal]

Resolves microsoft/vcpkg#24084
Resolves microsoft/vcpkg#34095

The root cause of this bug is the separation between the BaselineProvider and the PortfileProvider. For the built in registry, to determine what the baseline version for the builtin registry is, we must load and parse the requested port to read what version is declared in its JSON/CONTROL file. However, the interface by which the registry tells the calling code what to do passed only the directory with the port inside:

    struct RegistryEntry
    {
        // This forces BuiltinRegistry to parse the JSON/CONTROL
        virtual ExpectedL<View<Version>> get_port_versions() const = 0;
 
        // This forces the caller to parse the JSON/CONTROL
        virtual ExpectedL<PortLocation> get_version(const Version& version) const = 0;
 
        virtual ~RegistryEntry() = default;
    };

Thus, callers were forced to parse the files again, causing reemission of any warnings like bad SPDX expressions.

The fix is to push the parsing of the port files into RegistryEntry so that the builtin registry can return a clone of the file it already parsed, rather than getting it to be parsed again.