clarify API documentation

[szkiba]

Analyze() can detect dependencies from several sources (script, manifest file, environment variable). The Script source documentation is not detailed enough and therefore causes misunderstanding.

The Script option is a Source type struct, in which Name contains the name of the script file, and Contents contains the contents of the script. The script included in the Contents must be in CommonJS format and must not contain external script references (neither local nor remote), it can only refer to k6 extensions. That is, the script is bundled. The script should contain source map information to aid debugging.

If Contents is empty, loading and bundling is performed by Analyze(), otherwise bundling is the responsibility of the caller.