Minor refactoring of secret output code

[agateau-gg]

Context

While working on the SARIF output handler I came upon some code in the output which could be cleaner, so I made some minor refactoring.

What has been done

All the changes are detailed in individual commit. Some are very benign. The most important one is the renaming of MatchIndices to MatchSpan and the associated changes in it.

Validation

Tests still pass.

PR check list

• [x] As much as possible, the changes include tests (unit and/or functional)
• [x] If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[agateau-gg]

Thanks for the refacto! Just for my knowledge, is it standard practice to have Enum class be uppercase ?

There are no entries for them in PEP-8, but if you look at the official Enum doc (for example the Enum HOWTO), values are all uppercase, and the howto says:

Case of Enum Members

Because Enums are used to represent constants, and to help avoid issues with name clashes between mixin-class methods/attributes and enum names, we strongly recommend using UPPER_CASE names for members, and will be using that style in our examples.

So I'd say yes.