Small fixes for some issues with modules builds

[kamrann]

1. I think some recent changes caused a regression by rearranging some code such that some out-of-line member function definitions ended up in the module export scope. The errors encountered (on Clang) were as follows:

In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt\src\fmt.cc:107:
In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt/args.h:17:
In file included from C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt\format.h:41:
C:\Coding\Build2\third-party-packaging\build2-packaging\fmt\fmt\fmt/include\fmt\base.h:2471:41: error: cannot export 'do_check_arg_id' as it is not at namespace scope
 2471 | FMT_CONSTEXPR void parse_context<Char>::do_check_arg_id(int arg_id) {
      |                    ~~~~~~~~~~~~~~~~~~~~~^

2. Unsure if this was a recent regression or not, but the MSVC-specific inclusion of intrin.h was being done within the module purview.

[vitaut]

Thank you!