autofill: Isolate generated country_native_names_inl.h into source_set

Moves geo/country_names.cc (the only file including
country_native_names_inl.h) into an internal source_set with public = [].

Previously, all ~250 C++ files in //components/autofill/core/browser:browser
inherited an input dependency on :generate_country_native_names, which
waits on country_native_names_generator -> libbase_i18n.so -> Rust ICU
crates. Isolating this generated header unblocks Autofill C++
compilation ~84 seconds earlier on clean builds.

TAG=agy
CONV=a26e41c2-8955-4e98-931a-4f1c1674383a

Bug: 563098656
Change-Id: I4d74cc790eeab8b6a5945810b0f7ea9a6249e611
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8429103
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1709817}