[Fix]: only detect functions that return JSX somewhere as components

[jzabala]

Registering all functions that return JSX + null as components has bring a lot of false positives for normal functions that just happen to return null somewhere.

In this PR I propose a new approach to detecting components by finding all return statements in a function and only marking the function as a component if one of them returns JSX.

Fixes #2010

[ljharb]

In React 16, returning an array, a string, or a number (and I think a boolean) are all valid components.

[jzabala]

In React 16, returning an array, a string, or a number (and I think a boolean) are all valid components.

Yeah, but adding those types in my opinion will bring even more false positives that the null case.

By the way the only meaningful thing that this PR those is to remove the null check and collect all return statements from functions. The isReturningJSX algorithm is the same. That is why almost no test was broken.

[jzabala]

Closed based in #2707 (comment)