Thsi diagnostic currently emits, for example:
```
could not find module Foo for target arm64; found: x86_64
```
It is sometimes very useful to know where exactly the `found` module is located, so this PR changes this diagnostic to emit:
```
could not find module Foo for target arm64; found: x86_64, at:
<Path where Foo.swiftmodule/x86_64.swiftmodule is located>
```
With this change, swiftc will still look for standard library and overlay modules in ../lib/swift (relative to the compiler), but if it doesn’t find them there, it will now look in usr/lib/swift in the SDK.
