Add example of mixed-source app and fix module name bug

[ob]

Add a mixed source example and get it closer to compiling.

Without this PR, I get:

> bazel build //examples/...
INFO: Analyzed 15 targets (2 packages loaded, 53 targets configured).
INFO: Found 15 targets...
ERROR: /Users/obonilla/o/rules_ios/examples/mixed-test/BUILD.bazel:3:1: Compiling Swift module mixed-test failed (Exit 1)
<unknown>:0: error: module name "mixed-test" is not a valid identifier
INFO: Elapsed time: 0.180s, Critical Path: 0.05s
INFO: 4 processes: 4 darwin-sandbox.
FAILED: Build did NOT complete successfully

After normalizing the module name, I get:

> bazel build //examples/...
DEBUG: /Users/obonilla/o/rules_ios/rules/library.bzl:403:9: module name = mixed_test
INFO: Analyzed 15 targets (2 packages loaded, 53 targets configured).
INFO: Found 15 targets...
ERROR: /Users/obonilla/o/rules_ios/examples/mixed-test/BUILD.bazel:3:1: C++ compilation of rule '//examples/mixed-test:mixed-test_objc' failed (Exit 1) wrapped_clang failed: error executing command external/local_config_cc/wrapped_clang -arch x86_64 '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -O0 -DDEBUG -include ... (remaining 50 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
examples/mixed-test/ObjcFile.m:12:9: fatal error: 'mixed_test-Swift.h' file not found
#import "mixed_test-Swift.h"
        ^~~~~~~~~~~~~~~~~~~~
1 error generated.
INFO: Elapsed time: 4.883s, Critical Path: 4.76s
INFO: 9 processes: 6 darwin-sandbox, 3 worker.
FAILED: Build did NOT complete successfully

This is because the name of the module is actually mixed_test_swift-Swift.h. Unfortunately, rules_swift's swift_library rule doesn't allow changing the name of the header generated (see this PR for a possible solution there).

Putting this up here while we decide whether to push on that swift_library PR and bump our dependency (preferred) or work around it.

[segiddins]

I think we should only normalize in the fallback case, i.e.

kwargs.pop("module_name", _normalize_module_name(name))

[ob]

Hmm... good point. I'll change it.

[segiddins]

needs an import Foundation

[ob]

Shouldn't that come from the Objective-C headers?

[segiddins]

I don't think Swift will pull in transitive modules like that?