Add github URIs

For example, github:edolstra/dwarffs is more-or-less equivalent to
https://github.com/edolstra/dwarffs.git. It's a much faster way to get
GitHub repositories: it fetches tarballs rather than entire Git
repositories. It also allows fetching specific revisions by hash
without specifying a ref (e.g. a branch name):

  github:edolstra/dwarffs/41c0c1bf292ea3ac3858ff393b49ca1123dbd553

src/libexpr/primops/flake.cc

@@ -68,7 +68,8 @@ struct Flake
     Value * vProvides; // FIXME: gc
 };
 
-std::regex flakeRegex("^flake:([a-zA-Z][a-zA-Z0-9_-]+)$");
+std::regex flakeRegex("^flake:([a-zA-Z][a-zA-Z0-9_-]*)(/[a-zA-Z][a-zA-Z0-9_.-]*)?$");
+std::regex githubRegex("^github:([a-zA-Z][a-zA-Z0-9_-]*)/([a-zA-Z][a-zA-Z0-9_-]*)(/([a-zA-Z][a-zA-Z0-9_-]*))?$");
 
 static Path fetchFlake(EvalState & state, const std::string & flakeUri)
 {
@@ -76,6 +77,7 @@ static Path fetchFlake(EvalState & state, const std::string & flakeUri)
 
     if (std::regex_match(flakeUri, match, flakeRegex)) {
         auto flakeName = match[1];
+        auto revOrRef = match[2];
         auto registry = state.getFlakeRegistry();
         auto i = registry.entries.find(flakeName);
         if (i == registry.entries.end())
@@ -83,6 +85,25 @@ static Path fetchFlake(EvalState & state, const std::string & flakeUri)
         return fetchFlake(state, i->second.uri);
     }
 
+    else if (std::regex_match(flakeUri, match, githubRegex)) {
+        auto owner = match[1];
+        auto repo = match[2];
+        auto revOrRef = match[4].str();
+        if (revOrRef.empty()) revOrRef = "master";
+
+        // FIXME: require hash in pure mode.
+
+        // FIXME: use regular /archive URLs instead? api.github.com
+        // might have stricter rate limits.
+        auto storePath = getDownloader()->downloadCached(state.store,
+            fmt("https://api.github.com/repos/%s/%s/tarball/%s", owner, repo, revOrRef),
+            true, "source");
+
+        // FIXME: extract revision hash from ETag.
+
+        return storePath;
+    }
+
     else if (hasPrefix(flakeUri, "/") || hasPrefix(flakeUri, "git://")) {
         auto gitInfo = exportGit(state.store, flakeUri, {}, "", "source");
         return gitInfo.storePath;