followup #18362: make UnusedImport work robustly

[timotheecour]

• follows even lighter version of #17938: fix most issues with UnusedImport, XDeclaredButNotUsed, etc; fix #17511, #17510, #14246 (without realModule) #18362
• fixes pre-existing issues with UnusedImport which didn't work with re-exported symbols for cases like:

from a import b
import a except nil

• fix some false positive UnusedImport warnings introduced in even lighter version of #17938: fix most issues with UnusedImport, XDeclaredButNotUsed, etc; fix #17511, #17510, #14246 (without realModule) #18362 for cases like these: from a import b; a.foo
• reduce size of exportIndirections to only consider re-exports, not imported symbols; this is done through a new importModuleMap (which avoids adding a realModule: PSym field in the PSym ast)
• fix some UnusedImport and DuplicateModuleImport warnings in compiler sources
• warnDuplicateModuleImport => hintDuplicateModuleImport
• add comprehensive tests