Remaining places where "base" message want reviewing

[dra27]

This is a tracking issue for possible residual behaviour in 2.1 from opam 2.0's base packages which don't make sense in 2.1 with switch invariants.

• Base packages don't recompile with upstream changes - Rebuild "base" packages on changes #4569
• Pinning prompts about pinning base packages - Don't confirm pinning invariant packages #4571
• OpamClient.fixup - base message in there, does it just want rewording? - Update remaining references to base packages #4623
• OpamClient.install_t - updating the installed automatically state of base packages. That no longer matters? - Update remaining references to base packages #4623
• OpamSwitchCommand.install_compiler - "Could not resolve base install" and "base package installs". I expect these messages just need re-wording. - Update remaining references to base packages #4623

The remaining three items are not critical for 2.1.0. However, we should also consider:

Do we still need to care about the compiler flag in the same way?

[dra27]

Fixed with #4623