feat(ctx): Aspect contributions can access source and target contexts

[vic]

Usage Example:

# foo -> [bar] 
den.ctx.foo.into.bar = { foo }: [ { bar = 22; } ];

# aspect for configuring foo
den.ctx.foo.provides.foo = { foo }: <aspect>;

# aspect for configuring bar
den.ctx.bar.provides.bar = { bar }: <aspect>;

# NEW: foo contributes configuration to bar
# The aspect here can read from both source and target contexts
den.ctx.foo.provides.bar = { foo }: { bar }: <aspect>;