[swift-evolution] Beef up Imports

[Kevin Ballard]

I like the idea here, but I'm not sold on the syntax. I also do
explicitly want an `import qualified`. And with qualified imports, I
question whether we really need to support renaming in the import
syntax here.

I'm tempted to say we should just crib Haskell's import rules
(https://wiki.haskell.org/Import), with the minor change that importing
members by name still makes them accessible via the module name too (in
Haskell `import Mod (x, y)` makes `x` and `y` visible but does not make
`Mod.x` or `Mod.y` visible). This lets you say things like

*import* Mod // imports Mod and all its members *import* Mod () // only
provides access to protocol conformances declared in Mod, doesn't
actually *import* anything *import* Mod *(*x,y*)* // imports `x` and
`y`, which are also accessible as e.g. `Mod.x`, but does not provide `z`
or `Mod.z` *import qualified* Mod // imports Mod but all access to
members has to go through it, e.g. `Mod.x` *import qualified* Mod
*(*x,y*)* // imports Mod but only provides access to `Mod.x` and `Mod.y`
but not e.g. `Mod.z` *import *Mod *hiding* *(*x,y*)* // imports Mod and
its members except for `x` or `y` *import qualified* Mod *hiding*
*(*x,y*)* // imports e.g. `Mod.z` but not `Mod.x` or `Mod.y`* * *import*
Mod *as* Foo // imports Mod and renames the module to Foo, so e.g. `x`
and `Foo.x` are accessible *import* Mod *as* Foo *(*x,y*)* // renames
Mod to Foo, provides `x`, `y`, `Foo.x`, and `Foo.y` *import qualified*
Mod *as* Foo // renames Mod to Foo, all members are accessible via the
module e.g. `Foo.x` *import qualified* Mod *as* Foo *(*x,y*)* // renames
Mod to Foo, provides access to `Foo.x` and `Foo.y` but not e.g. `Foo.z`

Furthermore, you can have multiple import statements from the same
module, so you can say something like

*import qualified* Mod *import* Mod *(*x,y*)*

to provide access to all of Mod qualified with the name, and
additionally import `x` and `y` as unqualified identifiers.

-Kevin Ballard
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