Properly deconflict ids of default exports that have been reassigned - #2502

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Resolves #2414

Description

See also #2414. The logic for handling default exports of function and class declarations did not properly take into account that those not only declare a local variable as well but that reassigning this local variable will reassign the default export. Cf. these examples:

// the default export will be 42 because it is the value of a that is exported
var a = 42;
export default a;
a = 43;

vs.

// the default export will return 43 because it is the binding of a that is exported
export default function a() {return 42;};
a = () => 43;

Previously, only the first case was handled properly while this PR also handles the second case right. This also cleans up some logic so that almost everything related to default exports with ids is now handled together in ExportDefaultVariable.