debugger: provide access to box lua sources

tarantool.debug.getsources() used to provide
lua sources only for libserver lua modules,
and was not providing sources for src/box/lua/*
modules.

Now this code works:
```lua
local tnt = require 'tarantool'
local code1 = tnt.debug.getsources('box/session')
local code1 = tnt.debug.getsources('@builtin/box/session..lua')
```

Closes #7839

NO_DOC=bugfix
NO_CHANGELOG=later

src/box/lua/init.c

@@ -39,6 +39,7 @@
 #include "lua/utils.h" /* luaT_error() */
 #include "lua/trigger.h"
 #include "lua/msgpack.h"
+#include "lua/builtin_modcache.h"
 
 #include "box/box.h"
 #include "box/txn.h"
@@ -563,6 +564,7 @@ box_lua_init(struct lua_State *L)
 			panic("Error loading Lua module %s...: %s",
 			      modname, lua_tostring(L, -1));
 		lua_pop(L, 1); /* modfile */
+		builtin_modcache_put(modname, modsrc);
 	}
 
 	assert(lua_gettop(L) == 0);

test/app-luatest/tnt_debug_getsources_test.lua

@@ -15,6 +15,14 @@ local files = {
     'fiber',
     'env',
     'datetime',
+    'box/session',
+    'box/tuple',
+    'box/key_def',
+    'box/schema',
+    'box/xlog',
+    'box/net_box',
+    'box/console',
+    'box/merger',
 }
 
 -- calculate reporsitory root using directory of a current
@@ -32,6 +40,9 @@ g.test_tarantool_debug_getsources = function()
     local lua_src_dir = fio.pathjoin(git_root, '/src/lua')
     t.assert_is_not(lua_src_dir, nil)
     t.assert(fio.stat(lua_src_dir):is_dir())
+    local box_lua_dir = fio.pathjoin(git_root, '/src/box/lua')
+    t.assert_is_not(box_lua_dir, nil)
+    t.assert(fio.stat(box_lua_dir):is_dir())
 
     local tnt = require('tarantool')
     t.assert_is_not(tnt, nil)
@@ -39,7 +50,14 @@ g.test_tarantool_debug_getsources = function()
     t.assert_is_not(luadebug, nil)
 
     for _, file in pairs(files) do
-        local path = ('%s/%s.lua'):format(lua_src_dir, file)
+        local box_prefix = 'box/'
+        local path
+        if file:match(box_prefix) ~= nil then
+            path = ('%s/%s.lua'):format(box_lua_dir,
+                                        file:sub(#box_prefix + 1, #file))
+        else
+            path = ('%s/%s.lua'):format(lua_src_dir, file)
+        end
         local text = readfile(path)
         t.assert_is_not(text, nil)
         local source = luadebug.getsources(file)