Silence errors on ambiguous implicitly public imports when a different import
is explicitly public. In this case all imports are public so the ambiguity is
not dangerous and this configuration can happen during adoption of
access-levels on imports. Keep the error when an explicit import is package,
internal or below.
rdar://129037503
Adoption InternalImportsByDefault provides a safe access-level by default
to imports, as such ambiguities are not a risk and showing this warning is
superflous. When this warning is shown, make sure we note this alternative.
In Swift 5 mode, the compiler reports when two files import the same
module, where one file has a bare import and the other an access-level
import. This avoids unintentionally promoting the import to public when
preexisting code marked it otherwise.
Exempt a file generated by Xcode from this check as it's not user
modifiable.
In the future, we should identify such generated files properly. Either
with a new attribute or in the way they are passed to the compiler.
rdar://122032472
The feature InternalImportsByDefault makes imports default to internal instead
of public. Applying the Swift 6 behavior of SE-0409 in Swift 5.
Let's use only that flag to track the Swift 6 behavior as well instead
of separately checking for the language version.
When targeting Swift 5 and earlier, the default import is public. To
notify of a possible unintentional public import, raise errors on
default imports when other imports of the same target have an explicit
type.
This check isn't necessary in the tentative Swift 6 mode.
