bug(import/namespace): (failing test available) namespace + variable declaration merge "not found in imported namespace"

[llllvvuu]

Here is a popular file that breaks (16 million weekly downloads): https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/body-parser/index.d.ts

Failing test case: #2860

The docs say:

eslint-plugin-import/docs/rules/namespace.md

Lines 26 to 28 in cd95728

	Currently, this rule does not check for possible
	redefinition of the namespace in an intermediate scope. Adherence to the ESLint
	`no-shadow` rule for namespaces will prevent this from being a problem.

no-shadow doesn't apply here (it shouldn't - this file successfully compiles and declaration merges under tsc), since it's a declaration merge and not a shadow. In the case of @types/body-parser, a variable + namespace declaration merge is necessary (DefinitelyTyped/DefinitelyTyped#57168), since deprecating function overloads is weird in TypeScript: microsoft/TypeScript#40007.

Also, even if it were a shadow, it's a bit harder to adhere when it's in a third-party package.

I could look into adding merging to:

eslint-plugin-import/src/ExportMap.js

Line 306 in cd95728

ExportMap.for = function (context) {

Actually merging and redeclaration should have the same implementation in this case, since tsc only allows merging when the namespace has no fields.

Another option could be to deprecate "not found in imported namespace" message since it's a slightly complex feature and I think TypeScript handles it better.