Use pointer type for right use of mutex

[morikuni]

Hi, I found a bug in a default SourceCodeLoader.

fsLoader uses sync.Mutex, but its receiver type of Load is a value.
So mu.Lock and mu.Unlock does not affects shared state.
It causes race condition when writing fsLoader.cache from multiple goroutines.

[mattrobenolt]

Thanks for the fix! 🍕

[pierrre]

Thank you ! 👍
I just had a panic/crash in production today concurrent map writes.
I think it will fix it.