Improving extraction of dependencies.

This commit fixes the following bugs:

- Dynamic dependencies were not properly ignored in some
cases. In the following example, the dependency extractor
no longer returns "./path/" as a dependency:
    require("./path/" + somethingDynamic)

- Calls to require which contained comments were not properly
handled. In the following example, the dependency extractor
no longer ignores "myDependency":
    require(/* comment */ "myDependency" /* other comment */);