Improve InitialLoadContext loadedResources key - for unique loaded resources

[rbygrave]

This is in preparation for using the existing InitialLoadContext.loadedResources set to determine if a resource has already been loaded.

[agentgt]

To relate how this would change if we did the #176 but where:

public @Nullable KeyValuesSource resolve(URI uri, ... some other stuff like where the URI came from and env) {
}

public KeyValues load(KeyValuesSource source) {
}

In our system there is not just File and Classpath Sources and they are not a singleton.

KeyValuesSource has flags (and methods but doing an EnumSet is easier than adding tons of boolean methods) where you can test whether it is unique.

KeyValuesSource source = resolve(...);
if (source.isUnqiue()) { // or check some enum is set like isFlag(SomeEnum) this is the LoadFlags stuff I have talked about in the past.
}

EDIT to make it clear there would be (replace KeyValues with Config if you like).

• FileKeyValuesSource
• ClasspathKeyValuesSource

These get resolved either by URI or by programmatically creating them.

EDIT again this is just to give ideas based on what we did:

	public interface KeyValuesSource {
		public URI getUri();
		public @Nullable KeyValue getReference();
		public @Nullable MediaType getMediaType();
		public KeyValues getParameters();
		public Set<LoadFlag> getLoadFlags(); }

The parser is

public interface KeyValuesParser {

	public Iterable<KeyValue> parse(
			KeyValuesSource source,
			InputStream is)
			throws IOException;
}

There is some screwed up instanceof which I would have done with sealed classes where a KeyValuesSource can do the loading itself because it implements KeyValuesLoader and thus does not need to be parsed (e.g. environment variables for example).

public interface KeyValuesLoader {
   public Iterable<KeyValue> load();
}

If a KeyValuesSource implements KeyValuesLoader we don't do the parsing.

A KeyValue unlike Entry<String,String> has more information like where it came from and what it was before it was interpolated etc.

cc @SentryMan