start using "go install pkg@version" in docs

Now that Go 1.17.x is out,
we no longer need to worry about users on Go 1.15.x.

Since Go 1.16, the best way to install programs has been "go install":
https://golang.org/doc/go1.16#go-command

This method does not interfere with the current module,
and allows selecting a version such as "latest" or "master".

README.md

@@ -1,6 +1,6 @@
 # garble
 
-	GO111MODULE=on go get mvdan.cc/garble
+	go install mvdan.cc/garble@latest
 
 Obfuscate Go code by wrapping the Go toolchain. Requires Go 1.16 or later.
 
@@ -44,7 +44,7 @@ Note that commands like `garble build` will use the `go` version found in your
 and set up `$PATH` with them. For example, for Go 1.16.1:
 
 ```sh
-$ go get golang.org/dl/go1.16.1
+$ go install golang.org/dl/go1.16.1@latest
 $ go1.16.1 download
 $ PATH=$(go1.16.1 env GOROOT)/bin:${PATH} garble build
 ```

shared.go

@@ -30,12 +30,12 @@ type sharedCache struct {
 	// useful for type checking of the packages as we obfuscate them.
 	ListedPackages map[string]*listedPackage
 
-	// We can't rely on the module version to exist, because it's
-	// missing in local builds without 'go get'.
+	// We can't rely on the module version to exist,
+	// because it's missing in local builds without 'go install'.
 	// For now, use 'go tool buildid' on the garble binary.
 	// Just like Go's own cache, we use hex-encoded sha256 sums.
-	// Once https://github.com/golang/go/issues/37475 is fixed, we
-	// can likely just use that.
+	// Once https://github.com/golang/go/issues/37475 is fixed,
+	// we can likely just use that.
 	BinaryContentID []byte
 
 	// From "go env", primarily.