[module.import]/3 remove redundant importing of interface partitions

[RedBeard0531]

https://eel.is/c++draft/module.import#7 already describes how importing a TU implicitly imports all TUs that are export imported from the TU. https://eel.is/c++draft/module.unit#3.sentence-3 ensures that all interface partitions are transitively exported by the primary interface unit. Therefore, [module.import]/3 does not need to say that it imports "all module interface units of M" (unless the implication is that it also imports interface partitions that are not transitively exported by the PMI). I think it is simpler if [module.import]/3 only describes the "root" interface TU being imported with /7 handling the transitive importing of partitions. This also matches the wording used at https://eel.is/c++draft/module.unit#8 to describe the implicit import M; that occurs after module M;.

[jensmaurer]

@zygoloid , what's your opinion here?

[zygoloid]

I think what a reader of this section wants to know is that importing a module effectively imports all module interface units -- that's the intended semantic effect, and transitive re-export via export import :foo is more of a vehicle by which we get that effect. While I think it's sufficient and arguably more elegant to say that we import just the primary interface unit here, if we make this change I think we should add a note saying that as a consequence, all interface partitions are also imported.