[#1578] Minor Makefile fixes

- Add VERSION file to make it work without .git folder
- Remove obsolete GO111MODULE flag
- Move linter version to variable
- clean .cache on `make clean`
- Move help target to a separate help.mk file

Signed-off-by: Stanislav Bogatyrev <stanislav@nspcc.ru>

help.mk

@@ -0,0 +1,11 @@
+.PHONY: help
+
+# Show this help prompt
+help:
+	@echo '  Usage:'
+	@echo ''
+	@echo '    make <target>'
+	@echo ''
+	@echo '  Targets:'
+	@echo ''
+	@awk '/^#/{ comment = substr($$0,3) } comment && /^[a-zA-Z][a-zA-Z0-9_-]+ ?:/{ print "   ", $$1, comment }' $(MAKEFILE_LIST) | column -t -s ':' | grep -v 'IGNORE' | sort | uniq