[SizzleUnrlsd]

Summary

Robustness fixes found by running the analyzer on real repositories.

• Tests first (test: define discovery of virtual environments and non-identifier module names), fixes follow.
• discover_sources skips any virtual environment, recognised by the pyvenv.cfg the venv module writes at its root, whatever the directory is called (env/, tools/), on top of the fixed list of tooling directories. A Windows venv named env/ with 2,570 files was being analysed.
• project_symbol tolerates module and package names that are not Python identifiers (network-topologer, docutils-script.py): invalid characters become underscores and a leading digit gets a prefix, so the project index stays valid instead of the engine raising ValueError and aborting the whole run. Such modules cannot be imported by that name anyway, so nothing else resolves to them.

Found on the Pylinkit and network-topologer repositories; both now analyse to completion.