feat: support components with fully-qualified names with @mtkmodel

With the `@named` macro, we could use fully-qualified names for
components. However, trying to do the same with `@mtkmodel`,
```
@mtkmodel Model begin
     @components begin
          resistor = ModelingToolkitStandardLibrary.Electrical.Resistor(R = 1)
          ...
     end
     ...
end
```
throws the following error.
```
ERROR: LoadError: MethodError: Cannot `convert` an object of type Expr to an object of type Symbol

Closest candidates are:
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:84
  Symbol(::Any...)
   @ Base strings/basic.jl:229
```

Fix this and support fully qualified names by considering the
fully-qualifed name's Expr while parsing components.

Author: Anant Thazhemadam

src/systems/model_parsing.jl

@@ -275,13 +275,15 @@ function parse_components!(exprs, cs, dict, body, kwargs)
     expr = Expr(:block)
     varexpr = Expr(:block)
     push!(exprs, varexpr)
-    comps = Vector{Symbol}[]
+    comps = Vector{Union{Symbol, Expr}}[]
     for arg in body.args
         arg isa LineNumberNode && continue
         MLStyle.@match arg begin
             Expr(:(=), a, b) => begin
                 push!(cs, a)
-                push!(comps, [a, b.args[1]])
+                if(isa(b.args[1], Symbol) || Meta.isexpr(b.args[1], :.))
+                    push!(comps, [a, b.args[1]])
+                end
                 arg = deepcopy(arg)
                 b = deepcopy(arg.args[2])