Move the command to cmd/sfdupes and the code to internal/sfdupes (closes #157)

The root held every .go file in package main, so no ref had the
path cmd/sfdupes that go install was given. cmd/sfdupes/main.go now
only calls sfdupes.Main; the rest is package internal/sfdupes, with
main.go renamed to cli.go. The Makefile builds ./cmd/sfdupes and sets
Version by its new import path. README and TODO.md give the new
install path and file paths.

The test files carry //nolint:testpackage: they test unexported
functions, which the linter allowed only in package main.

Model: opus-5-5

Makefile

@@ -4,7 +4,7 @@ export CGO_ENABLED = 0
 
 BINARY := sfdupes
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
-LDFLAGS := -X main.Version=$(VERSION)
+LDFLAGS := -X sneak.berlin/go/sfdupes/internal/sfdupes.Version=$(VERSION)
 
 .PHONY: sfdupes build bootstrap setup test lint fmt fmt-check check docker hooks clean
 
@@ -14,7 +14,7 @@ LDFLAGS := -X main.Version=$(VERSION)
 # Default target: build the binary. Phony so go build (which has its
 # own build cache) always decides what to recompile.
 sfdupes:
-	go build -ldflags "$(LDFLAGS)" -o $(BINARY)
+	go build -ldflags "$(LDFLAGS)" -o $(BINARY) ./cmd/sfdupes
 
 build: sfdupes