Fix detection of export syntax in declaration file

[gabritto]

When we inspect a declaration file, we try to decide if it uses export = syntax or named/default ("normal") exports. To do this, we were looking for export constructs in the AST. If we fail to find any export construct, then we stop (with an inferenceError) and don't proceed to infer the type of the module and so we don't perform any further checks.
However, if there's no explicit export keyword or statement, every declaration is considered as exported. So in cases where there's no explicit export, we don't check for instance if the properties exported from the JS match the d.ts.
This PR changes the code to always consider that we're dealing with "normal" exports if we don't find an export =, so we're able to perform further checks for more packages.
The observable result should be that we're able to perform the "JsPropertyNotInDts" check for more packages, which only changes the suggestions provided and doesn't interfere with linting, but it would be nice to test this and future PRs using dtslint-runner before merging and publishing.