ast,fmt: further improve type detection of user defined typenames, extend tests

[ttytm]

Further improvements so that...

• a mod.submod.Type specification is automatically formatted

  // ~/.vmodules/foo/foo.v || ~/.vmodules/foo/src/foo.v
  module foo
  
  import bar.baz
  
  type MyAlias = bar.baz.BarBaz
  // Will now be formatted to
  type MyAlias = baz.BarBaz

  // ~/.vmodules/foo/bar/baz/baz.v
  module baz

• the module (mod.Type[...[]...]) is included in error messages for arrays in generics when using imported datatypes

  current: error: cannot cast struct `BSTree[Result[[]Token, Err[string]]]` to `int`
  update: error: cannot cast struct `datatypes.BSTree[Result[[]Token, Err[string]]]` to `int`
  

  That is uniform with the behavior for imported types in other error messages. E.g.:
  

  v/vlib/v/checker/tests/fn_mismatch_option_return_err.out

  Line 1 in aeadc0a

  vlib/v/checker/tests/fn_mismatch_option_return_err.vv:4:15: error: cannot use `?time.Duration` as type `time.Duration` in return argument

  
  

  v/vlib/v/checker/tests/import_symbol_private_err.out

  Line 1 in aeadc0a

  vlib/v/checker/tests/import_symbol_private_err.vv:13:6: notice: interface field `io.ReaderWriterImpl.r` must be initialized