fix multiple time imports using reference variables

[pagran]

Alternate realization of fix.

Сurrent version redesigns logic for handling unnecessary (after shrink runtime or literals).
Now instead of renaming unnecessary imports, it creates global unnamed variables referencing to declaration inside unused import (note: go/types returns sorted decl's)

If decl is constant, variable or function:

var _ = <target decl>

If decl is type (struct, type, interface, etc...):

var _ <target decl>

Fix: #658

[pagran]

Referencing to types is not safe (generics, type constraint). How about referring to garbleActionID (making it exportable first)? @mvdan

upd: At second look this idea is not so good, for example unsafe package does not have garbleActionId
upd: For now, i add checking for unsafe declarations, but it is still possible in extreme cases (in theory, if generics appear at runtime)
upd2: It's harder than I thought

[mvdan]

Right, you could always have a package which doesn't export any names, if it were imported like _ "pkg/path" for example. And in other cases, I agree that generics can make it awkward.

FWIW I think any exported name works except generic types. For regular types, use var _ pkg.Name. For anything else (funcs, vars, consts), use var _ = pkg.Name. If any package doesn't have any of these, we can for now assume that it won't become unused, which is probably right. A package only becomes unused with -literals when it has a constant being used that we replace, for example.

[mvdan]

Relying on garbleActionID is also clever, but I don't think it's a good idea in general. It relies on us being able to modify all packages to insert the var, and inserting the var has always been a hack that I'd like to get rid of at some point.

[mvdan]

Looks good - some thoughts to polish this up.