Swift: use std::filesystem and picoSHA2

This replaces usages of `llvm::fs` and string manipulation with
`std::filesystem`, also replacing `std::string` with
`std::filesystem::path` where it made sense.

Moreover MD5 hashing used in macOS file remapping was replaced by
SHA256 hashing using a small header-only SHA256 C++ library with an
MIT license, https://github.com/okdshin/PicoSHA2.

File contents hashing was relocated to the newly created `file` library
for later planned reuse.

swift/extractor/TargetTrapFile.cpp

@@ -2,8 +2,8 @@
 #include <iomanip>
 namespace codeql {
 std::optional<TargetFile> createTargetTrapFile(const SwiftExtractorConfiguration& configuration,
-                                               std::string_view target) {
-  std::string trap{target};
+                                               const std::filesystem::path& target) {
+  auto trap = target;
   trap += ".trap";
   auto ret = TargetFile::create(trap, configuration.trapDir, configuration.getTempTrapDir());
   if (ret) {