luci-lib-base: ensure that `luci.http` can be required standalone

Various existing Lua software is requiring the `luci.http` library for
URL encoding/decoding tasks so ensure that it can be loaded in a stand
alone manner even if the emulated Lua runtime environment is not
available.

Fixes: cea2c357

 ("luci-lib-base: forward luci.http.context.request.message to ucode")
Ref: https://forum.openwrt.org/t/x/141817
Ref: https://github.com/openwrt/luci/pull/5976#issuecomment-1296220586
Signed-off-by: Jo-Philipp Wich <jo@mein.io>