fix: check scope when discovering variables from equations

[AayushSabharwal]

Before:

julia> function Foo(; name, p = 1)
           @parameters p = p
           @variables x(t)
           return ODESystem(D(x) ~ p, t; name)
       end
Foo (generic function with 1 method)

julia> function Bar(; name, p = 2)
           @parameters p = p
           @variables x(t)
           @named foo = Foo(; p)
           return ODESystem(D(x) ~ p + t, t; systems = [foo], name)
       end
Bar (generic function with 1 method)

julia> @named bar = Bar()
Model bar with 2 equations
Unknowns (2):
  x(t)
  foo₊x(t)
Parameters (1):
  p [defaults to 2]

After:

julia> @named bar = Bar()
Model bar with 2 equations
Unknowns (2):
  x(t)
  foo₊x(t)
Parameters (2):
  p [defaults to 2]
  foo₊p [defaults to p]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.