Thoroughly improve import resolution - #2584

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 #2576
resolves #2231
and quite possible a few more

Description

This is a huge follow-up to #2575. The original intention was to fix handling for dynamic namespaces when preserving modules which lead me down a deep rabbit whole and had me refactoring many core parts of the logic. This is what has been fixed and improved:

• Preserve modules now supports dynamic namespaces which is done not by creating namespace objects but by using a native way of doing a namespace import depending on the output format (i.e. import * as lib from ... for esm, const lib = require(...) for cjs and so on). Note that namespace freezing will not be performed when preserving modules!
• Shimming missing exports has been thoroughly re-worked and should now work properly for all export formats. Moreover, the shim variable will now always be deconflicted from existing variables of the same name!
• The SystemJS rendering code has been refactored and partially cleaned up to support proper missing export shimming.
• Export tracing has been thoroughly refactored. Most notably, there is no longer any tracing logic at all on chunk level! This was achieved by attaching the exporting module to all variables which also leads to better and more direct imports in some cases.
• The imports of a module are now directly determined during the tree-shaking phase with the awesome benefit that no unnecessary imports are rendered any longer! E.g if a.js and b.js both import a variable x from c.js that is only used in one of them, the other will no longer render the import! The effect becomes more pronounced when handling namespace imports. And the new logic is in fact a little simpler than the old one!