lib rules not included in scoped rules

[mr-tz]

This was observed in mandiant/capa-rules#225.

Thorough lints fail to validate lib rules, because we ignore "lib" rules here:
https://github.com/fireeye/capa/blob/20ce29b0333741c3f677fc501e3c7e34dc062077/capa/rules.py#L864-L879

Possible solutions:

• remove "lib" exclusion (easy, however, there may be undesired side-effects @williballenthin?)
• add RuleSet.lib_rules (not as easy)
• to only change the linter, set all rules' lib meta to False (hacky)

[mike-hunhoff]

This appears to be causing issues in the rule generator plugin as well e.g. when a user attempts to create a new lib rule the plugin cannot make a match because the rule isn't included in the final ruleset (as no dependencies yet exist):

With lib: true:

Without lib: true:

This also prevents existing lib rules with no dependencies from showing up in the rule generator under matched rules, even though these rules may match the current function.