fix: Fix local files with UTF8 names

[joeyparrish]

This fixes our use of std::filesystem to interpret all paths names as UTF8. Before this, UTF8 paths did not work correctly in all cases.

This also adds a new unit test to cover this case.

On Windows, it is critical that a UTF8 locale be set at runtime. Applications linking with Packager as a library should call setlocale(), and the Packager frontends now do this automatically after converting wide character arguments into narrow strings.

Closes #652

[cosmin]

LocalFileTest.UnicodePath seems to be failing on Windows.

[joeyparrish]

Yeah, I thought I had this right. file->Size() isn't working for some reason, only on Windows with Unicode paths, even though reading and writing is fine. I'll convert this back to draft while I iterate on it.

[joeyparrish]

Okay, this is finally working. I was able to verify it on a Windows machine in our lab before uploading this latest version. The critical bit is setlocale(), since we are explicitly interpretting narrow string paths as UTF-8. We need to set the locale to UTF8 for functions like fopen() to do the right thing with those narrow strings. We also need to setlocale() in wmain(), after converting wchar_t argv into UTF-8, for the same reason.