Convert is_login_page() to excavate rule

[TheTechromancer]

This PR converts our is_login_page() function, which was being used by httpx to detect login pages, to an excavate rule. In order to accomplish this, excavate was converted to an intercept module.

Making excavate an intercept module is the first step towards keeping a collection of simpler excavate rules as YAML files. By making it an intercept module, it now has the ability to tag events on-the-fly, before they're distributed to other modules.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93%. Comparing base (b5587df) to head (e81a549).
Report is 33 commits behind head on dev.

Additional details and impacted files

@@          Coverage Diff           @@
##             dev   #1844    +/-   ##
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- Coverage     93%     93%    -0%     
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  Files        350     354     +4     
  Lines      27274   27398   +124     
======================================
+ Hits       25166   25263    +97     
- Misses      2108    2135    +27     

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