[dev.regabi] cmd/compile: use ir.Ident for imported identifiers
This CL substantially reworks how imported declarations are handled,
and fixes a number of issues with dot imports. In particular:
1. It eliminates the stub ir.Name declarations that are created
upfront during import-declaration processing, allowing this to be
deferred to when the declarations are actually needed. (Eventually,
this can be deferred even further so we never have to create ir.Names
w/ ONONAME, but this CL is already invasive/subtle enough.)
2. During noding, we now use ir.Idents to represent uses of imported
declarations, including of dot-imported declarations.
3. Unused dot imports are now reported after type checking, so that we
can correctly distinguish whether composite literal keys are a simple
identifier (struct literals) or expressions (array/slice/map literals)
and whether it might be a use of a dot-imported declaration.
4. It changes the "redeclared" error messages to report the previous
position information in the same style as other compiler error
messages that reference other source lines.