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

@@ -673,7 +673,7 @@ function _parse_components!(exprs, body, kwargs)
     expr = Expr(:block)
     varexpr = Expr(:block)
     # push!(exprs, varexpr)
-    comps = Vector{Symbol}[]
+    comps = Vector{Union{Symbol, Expr}}[]
     comp_names = []
 
     for arg in body.args
@@ -692,7 +692,9 @@ function _parse_components!(exprs, body, kwargs)
                 arg.args[2] = b
                 push!(expr.args, arg)
                 push!(comp_names, 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
             end
             _ => error("Couldn't parse the component body: $arg")
         end