Allow custom modules to replace built-in modules (#902)

A custom module which extends a built-in one ran alongside it, and both
wrote to the same results file when they shared a slug, with the run
order deciding the surviving content.

Custom modules can now name the module class they supersede in a
`replaces` attribute. When both are available to a command, the named
module is dropped from the run and the substitution is logged with the
origin of the replacement, so it is recorded in command.log. Only the
replacements which are applied are reported: a declaration from a
disabled module is ignored, so that replacing a module cannot silently
disable it, and modules which replace each other in a cycle all keep
running and replace nothing, with a warning naming every one of them.

Dependencies are remapped along with the modules themselves: a module
depending on a replaced class is ordered against, and receives the
results of, the module which took its place. Replacing a module which
others depend on therefore does not make that dependency unavailable.

The remapping applies wherever dependencies are read, so a replacement
which cannot run is skipped like any other module with an unavailable
dependency, and takes the modules depending on the module it replaced
with it. Those warnings name the dependency the author declared as well
as the module which replaces it.

A replacement does not have to keep the class name of the module it
replaces, so `--module` now falls back to the name of a replaced module
and runs its replacement. A name which matches no module at all stops
the run with a warning instead of silently analyzing nothing, as does a
selection left with nothing to run once skipped modules are dropped.

Sharing a slug outside a replacement stays possible and is now reported.
Two modules writing to the same results file is a forensic-integrity
problem rather than an error, so both still run and a warning names them,
where each came from, and the file the later one overwrites. Taking over
the slug of a replaced module is not reported, because that module is no
longer part of the run.

docs/development/index.md

@@ -176,6 +176,59 @@ not, MVT skips the module with a warning.
 They are the same records it writes to `<slug>.json`. Typed results per module
 are planned.
 
+### Replacing a built-in module
+
+A custom module which extends a built-in one runs alongside it, and when the
+two share a slug they write to the same results file. MVT warns about that
+collision, naming both modules, where each came from and the file the later one
+overwrites, but it runs them both. Set `replaces` to the class the module
+supersedes to take that module's place instead: when both are available to a
+command, the named module is dropped from the run.
+
+```python
+from mvt.ios.modules.backup import Manifest as BuiltinManifest
+
+
+class Manifest(BuiltinManifest):
+    supported_commands = (("ios", "check-backup"),)
+    replaces = BuiltinManifest
+```
+
+Keeping the class name of the replaced module, as above, keeps the name
+`--module` selects the module by and the slug its results file is named after.
+A replacement with a different class name writes to a file named after its own
+slug, unless it sets `slug` to the slug of the module it replaces, and
+`--module` still selects it by the name of the replaced module, which MVT logs.
+Taking over the slug of a replaced module is not a collision and is not warned
+about, because that module is no longer part of the run.
+
+Modules which depend on the replaced module receive the replacement instead, so
+`get_dependency_results(BuiltinManifest)` returns the replacement's results. A
+module must not depend on the module it replaces: that module does not run, so
+the replacement has to produce the data itself, and MVT logs a warning when a
+module declares both.
+
+A replacement takes on the obligations of the module it replaces. If it
+declares a dependency the command does not provide, it is skipped like any
+other module with an unavailable dependency, and the module it replaced stays
+out of the run: neither of them produces results, and the modules depending on
+the replaced module are skipped as well.
+
+Subclassing the replaced module is not required, but a replacement which is not
+a subclass is logged with a warning, because its results may not be what the
+modules depending on the replaced module expect. Naming a module the command
+does not run has no effect, and modules which replace each other in a cycle all
+keep running and replace nothing. Every applied substitution is logged, so it
+is recorded in `command.log` when the command runs with an `--output` folder.
+
+Every command resolves replacements on its own. `check-iocs` matches stored
+results files against the slugs of the modules available for that command, so a
+replacement checks the indicators of its own results only if it also declares
+the `("ios", "check-iocs")` pair; otherwise the built-in module it replaced
+re-checks the file. It also matches `--module` on the class name only, so pass
+a differently named replacement's own name there, not the name of the module
+it replaces.
+
 ### Importing from MVT
 
 Import from `mvt.plugin` if it has what you need. The names it exports are