[no-unused-var] false positive with nested namespace and named function expression

[unional]

Repro

export namespace foo.config {
  export type Options = {}
}
export function foo() { }
foo.config = function config(options: foo.config.Options) { return options }

Expected Result
no error

Actual Result

Additional Info
The following works fine

export namespace foo.config {
  export type Options = {}
}
export function foo() { }
// anonymous function expression
foo.config = function (options: foo.config.Options) { return options }

export namespace foo.config {
  export type Options = {}
}
export function foo() { }
// function declaration
function config(options: foo.config.Options) { return options }
foo.config = config

Versions

package	version
@typescript-eslint/eslint-plugin	2.28.0
@typescript-eslint/parser	2.28.0
TypeScript	3.8.3
ESLint	6.8.0
node	12.14.1
npm	6.13.4

[bradzacher]

See #1856