[HW] Moved and renamed arc/inlineModules to hw/flattenModules

[dobios]

This PR moves the arc::inlineModulesPass to the HW dialect, all while renaming it to something more suitable for what it actually does, as per @fabianschuiki 's recommendation.

[uenoku]

Refactoring looks great to me! InlineModules pass currently doesn't handle inner symbols so can we open a tracking issue?

$ cat test.mlir
  hw.hierpath @hier [@Foo::@bar, @Bar::@a]
  hw.module private @Bar(in %a : i1 {hw.exportPort = #hw<innerSym@a>}, out b : i1) {
    hw.output %a : i1
  }
  hw.module @Foo(in %a : i1, out b : i1) {
    %bar.b = hw.instance "bar" sym @bar @Bar(a: %a: i1) -> (b: i1)
    hw.output %bar.b : i1
  }

$ circt-opt test.mlir -hw-flatten-modules -hw-verify-irn
test.mlir:1:3: error: 'hw.hierpath' op  module: "Foo" does not contain any instance with symbol: "bar"
  hw.hierpath @hier [@Foo::@bar, @Bar::@a]  
  ^
test.mlir:1:3: note: see current operation: "hw.hierpath"() {namepath = [#hw.innerNameRef<@Foo::@bar>, #hw.innerNameRef<@Bar::@a>], sym_name = "hier"} : () -> ()