lua: allow to refuse caching in package.loaded

Use case: an override module is designed to replace a built-in module
only when the module is required from an application, not from the
platform itself.

Now it is possible:

```lua
-- override/fio.lua
local loaders = require('internal.loaders')

if loaders.initializing then
    loaders.no_package_loaded.fio = true
    return loaders.builtin.fio
end

return {
    whoami = 'override.fio',
}
```

Follows up #7774

NO_DOC=Planned as an internal API. See the previous commit.
NO_CHANGELOG=see NO_DOC

src/lua/loaders.lua

@@ -373,6 +373,27 @@ rawset(package, "search", search)
 rawset(package, "searchroot", searchroot)
 rawset(package, "setsearchroot", setsearchroot)
 
+local no_package_loaded = {}
+local raw_require = require
+
+-- Allow an override module to refuse caching in package.loaded.
+--
+-- It may be useful to return a built-in module into the platform,
+-- but the override module into application's code.
+_G.require = function(modname)
+    if no_package_loaded[modname] then
+        no_package_loaded[modname] = nil
+        package.loaded[modname] = nil
+    end
+    -- NB: This call is a tail call and it is important to some
+    -- extent. At least luajit's test suites expect certain error
+    -- messages from `require()` calls and from the `-l` option.
+    --
+    -- A non-tail call changes the filename at the beginning of
+    -- the error message.
+    return raw_require(modname)
+end
+
 return {
     ROCKS_LIB_PATH = ROCKS_LIB_PATH,
     ROCKS_LUA_PATH = ROCKS_LUA_PATH,
@@ -386,4 +407,15 @@ return {
     -- It is `true` during tarantool initialization, but once all
     -- the built-in modules are ready, will be set to `nil`.
     initializing = true,
+    raw_require = raw_require,
+    -- Add a module name here to ignore package.loaded[modname] at
+    -- next require(). The flag is dropped at the require() call.
+    --
+    -- This is intended to be used from inside a module to refuse
+    -- caching in package.loaded. Any other usage has unspecified
+    -- behavior (just manipulate package.loaded directly if you
+    -- need it).
+    --
+    -- Usage: loaders.no_package_loaded[modname] = true
+    no_package_loaded = no_package_loaded,
 }

test/app-luatest/override_misc_test.lua

@@ -65,3 +65,26 @@ g.test_initializing = function(g)
     local exp = expected_output('socket')
     t.assert_equals(res, exp)
 end
+
+g.test_no_package_loaded = function()
+    local LOAD_LIMIT = 3
+    local LOAD_ATTEMPTS = LOAD_LIMIT + 1
+
+    local load_cnt = 0
+    local foo = {whoami = 'foo'}
+    package.preload.foo = function()
+        local loaders = require('internal.loaders')
+        load_cnt = load_cnt + 1
+        if load_cnt < LOAD_LIMIT then
+            loaders.no_package_loaded.foo = true
+        end
+        return foo
+    end
+
+    for _ = 1, LOAD_ATTEMPTS do
+        require('foo')
+    end
+
+    package.preload.foo = nil
+    t.assert_equals(load_cnt, LOAD_LIMIT)
+end