Document the terminology related to `built-in`, `installed` and `embedded` module

[jcfr]

Do we have somewhere a definition of "installed" and "built-in" module? (not installed = in build tree? not built-in = provided by extension?)

_Originally posted by @lassoan in https://github.com/Slicer/Slicer/pull/6970#discussion_r1197307006_

[jcfr]

not installed = in build tree ?

Since extension can be installed in the build tree in the "special" folder <applicationHome>/<org-name-or-domain>/Extensions-12345, saying this is not exactly true.

not built-in = provided by extension?

Not exactly correct either as module can be specific using option like --additional-module-path/--additional-module-paths and in that care they are not installed using the extension manager.

---

I create a "mermaid" diagram to include in our documentation

Reference:

• https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/
• https://sphinxcontrib-mermaid-demo.readthedocs.io/en/latest/

_Originally posted by @jcfr in https://github.com/Slicer/Slicer/pull/6970#discussion_r1197318853_

[jcfr]

I guess what I miss is a very high-level description of what installed and built-in flags are meant to distinguish between.

For example, built-in could refer to "not being manually installed by the user" or "source code is in Slicer source tree". Both could make sense, both could be useful to know.

_Originally posted by @lassoan in https://github.com/Slicer/Slicer/pull/6970#discussion_r1197327348_

[jcfr]

re: built-in

A built-in module is a module that is distributed along side the Slicer application.

This usually means its source code is either located in the Slicer source tree or integrated into it using add_subdirectory().

re: installed

A module is considered installed if it is not built-in and it it is installed using the extensions manager.

Generally speaking, this means it is loaded from an install tree.

re: embedded

Looking at the implementation, a module is considered embedded if it is a descendant of the <applicationHome> except <applicationHome>/<org-name-or-domain>/Extensions-12345

Conclusion

• There is no overlap between the set of built-in and installed modules.

• The set of all module loaded from install trees is the union of installed and built-in modules.

• Based on the current use of IsEmbeddedModule and its usage, I suggest to:

  • Remove the IsEmbeddedModule method
  • Update the method importModulePythonExtensions renaming its parameter isEmbedded to isBuiltIn.

Background to motivate the removal:

The function vtkSlicerApplicationLogic::IsPluginBuiltIn():

• was introduced in October 2015 through c32f83660
• is exhaustively tested
• fulfills the same role.

The function vtkSlicerApplicationLogic::IsEmbeddedModule():

• is not included in any unit tests
• was introduced in 2012 through e78f7bb9d after removing the function isExtension and mentioning that The concept of extension and module are decoupled, a module doesn't have to be an extension to be externally loaded..