Finish separating concerns with tarball cache

There is no longer an `importTarball` method. Instead, there is a
`unpackTarfileToSink` function (back in libutil). The caller can use
thisw with the `getParseSink` method we added in the last commit easily
enough.

In addition, tarball cache functionality is separated from `git-utils`
and moved into `tarball-cache`. This ensures we are separating mechanism
and policy.

src/libutil/tarfile.cc

@@ -132,4 +132,66 @@ void unpackTarfile(const Path & tarFile, const Path & destDir)
     extract_archive(archive, destDir);
 }
 
+time_t unpackTarfileToSink(TarArchive & archive, FileSystemObjectSink & parseSink)
+{
+    time_t lastModified = 0;
+
+    for (;;) {
+        // FIXME: merge with extract_archive
+        struct archive_entry * entry;
+        int r = archive_read_next_header(archive.archive, &entry);
+        if (r == ARCHIVE_EOF) break;
+        auto path = archive_entry_pathname(entry);
+        if (!path)
+            throw Error("cannot get archive member name: %s", archive_error_string(archive.archive));
+        if (r == ARCHIVE_WARN)
+            warn(archive_error_string(archive.archive));
+        else
+            archive.check(r);
+
+        lastModified = std::max(lastModified, archive_entry_mtime(entry));
+
+        switch (archive_entry_filetype(entry)) {
+
+        case AE_IFDIR:
+            parseSink.createDirectory(path);
+            break;
+
+        case AE_IFREG: {
+            parseSink.createRegularFile(path, [&](auto & crf) {
+                if (archive_entry_mode(entry) & S_IXUSR)
+                    crf.isExecutable();
+
+                while (true) {
+                    std::vector<unsigned char> buf(128 * 1024);
+                    auto n = archive_read_data(archive.archive, buf.data(), buf.size());
+                    if (n < 0)
+                        throw Error("cannot read file '%s' from tarball", path);
+                    if (n == 0) break;
+                    crf(std::string_view {
+                        (const char *) buf.data(),
+                        (size_t) n,
+                    });
+                }
+            });
+
+            break;
+        }
+
+        case AE_IFLNK: {
+            auto target = archive_entry_symlink(entry);
+
+            parseSink.createSymlink(path, target);
+
+            break;
+        }
+
+        default:
+            throw Error("file '%s' in tarball has unsupported file type", path);
+        }
+    }
+
+    return lastModified;
+}
+
 }