Rename stdlibErr to stdliberr or stderr

[yutongp]

for https://github.com/uber-go/fx/blob/master/ulog/log.go#L24

package-names: "Good package names are short and clear. They are lower case, with no under_scores or mixedCaps."

I like this convention because I know package names are all lower case so I can seperate them from private struct defined somewhere else. I think this convention applies to rename package as well.

So maybe 'stdliberr' or 'stderr'?

[ascandella]

@alsamylkin

[ghost]

https://github.com/uber-go/fx/pull/235