mv mathcomp_{extra,compat}

[affeldt-aist]

Motivation for this change

Rename mathcomp_extra.v to mathcomp_compat.v as discussed on zulip.
In the process, make the imports a bit more systematic.
For example, interval_inference.v is systematically put together with the MathComp imports,
not importing lebesgue_stieltjes_measure.v or constructive_ereal.v
when resp. lebesgue_measure.v or ereal.v is already imported.
Also it looks like interval_inference.v was exported by mistake,
this PR fixes it.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
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[affeldt-aist]

I am thinking of merging because there is at least one other PR that requires moving files around.