build: don't pack git-ignored files into tars

When temporary operating-system-specific files like .DS_Store
(a macOS Finder artifact) are packed into the dist/ tars, it
creates version control noise and can make the source-matches-tars
check fail unnecessarily.

This updates `make dist` to first `git clean` away any git-ignored
files. It also changes `make unpack` to work in the case where the
source directories are missing, which is useful when you want to
unpack the tars from scratch.

Finally, this commit re-runs `make dist`, so that the dist/
tars no longer have unnecessary OS-specific files in them.

.gitignore

@@ -2,4 +2,5 @@
 .DS_Store?
 .ehthumbs.db
 Icon?
-Thumbs.db.DS_Store
+Thumbs.db
+._*

Makefile

@@ -18,13 +18,16 @@ help: ## Display this help message.
 clean: ## Delete all generated course and library exports.
 	rm -f $(COURSE_TAR) $(LIBRARY_TAR)
 
-dist: ## Create/overwrite exports in ./dist/ folder for course and libary.
+dist: clean ## Create/overwrite exports in ./dist/ folder for course and libary.
 	cd $(COURSE) && tar czfv ../$(COURSE_TAR) ./course/
 	cd $(LIBRARY) && tar czfv ../$(LIBRARY_TAR) ./library/
 
+clean:  ## Remove hidden system files that are ignored by git.
+	git clean -dfX
+
 unpack: ## Unpack course and library exports from ./dist/ folder into source OLX.
-	[ -f $(COURSE_TAR) ] && (cd $(COURSE) && tar xzfv ../$(COURSE_TAR)) || echo "No course to unpack."
-	[ -f $(LIBRARY_TAR) ] && (cd $(LIBRARY) && tar xzfv ../$(LIBRARY_TAR)) || echo "No content library to unpack."
+	[ -f $(COURSE_TAR) ] && mkdir -p $(COURSE) && (cd $(COURSE) && tar xzfv ../$(COURSE_TAR)) || echo "No course to unpack."
+	[ -f $(LIBRARY_TAR) ] && mkdir -p $(LIBRARY) && (cd $(LIBRARY) && tar xzfv ../$(LIBRARY_TAR)) || echo "No content library to unpack."
 
 import: dist ## Import course and libraries into a locally-running Tutor instance. Requires an admin user.
 	$(TUTOR) mounts add cms,cms-worker:.:/openedx/data/$(REPO_NAME)