Fixed the relative resolver not checking if the file can be read in the

first place

packages/sol-resolver/src/resolvers/relative_fs_resolver.ts

@@ -14,7 +14,7 @@ export class RelativeFSResolver extends Resolver {
     // tslint:disable-next-line:prefer-function-over-method
     public resolveIfExists(importPath: string): ContractSource | undefined {
         const filePath = path.join(this._contractsDir, importPath);
-        if (fs.existsSync(filePath)) {
+        if (fs.existsSync(filePath) && !fs.lstatSync(filePath).isDirectory()) {
             const fileContent = fs.readFileSync(filePath).toString();
             return {
                 source: fileContent,