Work around npm pack issue with scoped packages

[noahtallen]

Resolves #46

Currently, it is not possible to run npm test @namespace/package locally for DefinitelyTyped (see DefinitelyTyped/DefinitelyTyped#56042). The error is a bug in npm, where it gives the incorrect filename for packed packages:

# Reported filename:
npm pack @wordpress/data-controls
# npm notice filename:      @wordpress/data-controls-2.2.6.tgz

# Actual filename:
ls
# wordpress-data-controls-2.2.6.tgz

Specifically, npm strips the leading @ and removes the / for the file it saves, but doesn't apply those changes to the filename it reports.

Dts critic relies on npm pack's reported filename. To work around this problem, if the downloaded tarball can't be found, we do the string replacement ourselves as a backup.