Refactor require() to make it easier to reason about Lua stack usage.

Push throwing Lua errors down into LLRequireResolver::findModule() and
findModuleImpl() so their callers don't have to handle the error case. That
eliminates finishrequire().

require() itself now only retrieves (and pops) the passed module name and
calls LLRequireResolver::resolveRequire() to do the actual work.

resolveRequire() is now void. It only instantiates LLRequireResolver and calls
its findModule().

findModule() is now also void. It's guaranteed to either push the loaded Lua
module or throw a Lua error. In particular, when findPathImpl() cannot find
the specified module, findModule() throws an error. That replaces
ModuleStatus::NotFound.

Since std::filesystem::path::append() aka operator/() detects when its right
operand is absolute and, in that case, discards the left operand, we no longer
need resolveAndStoreDefaultPaths(): we can just invoke that operation inline.

When findModule() pushes _MODULES on the Lua stack, it uses LuaRemover (below)
to ensure that _MODULES is removed again no matter how findModules() exits.

findModuleImpl() now accepts the candidate pathname as its argument. That
eliminates mAbsolutePath.

findModuleImpl() now returns only bool: true means the module was found and
loaded and pushed on the Lua stack, false means not found and nothing was
pushed; no return means an error was reported.

Push running a newly found module's source file down into findModuleImpl().
That eliminates the distinction between Cached and FileRead, which obviates
ModuleStatus: a bool return means either "previously cached" or "we read it,
compiled it, loaded it and ran it." That also eliminates the need to store the
module's textual content in mSourceCode.

Similarly, once loading the module succeeds, findModuleImpl() caches it in
_MODULES right away. That eliminates ResolvedRequire since we need not pass
the full pathname of the found module (or its contents) back up through the
call chain.

Move require() code that runs the new module into private runModule() method,
called by findModuleImpl() in the not-cached case. runModule() is the only
remaining method that can push either a string error message or the desired
module, because of its funny stack manipulations. That means the check for a
string error message on the stack top can move down to findModuleImpl().

Add LuaRemover class to ensure that on exit from some particular C++ block,
the specified Lua stack entry will definitely be removed. This is different
from LuaPopper in that it engages lua_remove() rather than lua_pop().

Also ditch obsolete await_event() Lua entry point.

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