Install from @latest, retract v0.0.1, print a go-installed binary's version (closes #116)

README "Install" now leads with go install ...@latest: until 1.0 each
release is a v0.0.N tag on main's tip, so @latest gives that tip once
it has passed the build. It also says what @main and v0.0.1 give and
how to check an installed binary. go.mod retracts v0.0.1, the early
build without a database.

sfdupes --version printed "dev" for every go-installed binary, since
only the linker set Version. When the linker sets none, it now prints
the main module's version from runtime/debug.ReadBuildInfo; "dev" is
left only when neither is known. Version is now empty by default.

The retraction takes effect only once a release carrying it is the
newest.

Model: opus-5-5

go.mod

@@ -24,3 +24,5 @@ require (
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect
 )
+
+retract v0.0.1 // early build without a database; does not work as the README describes