Move the main package to cmd/elvish.

Now that the module path is "src.elv.sh", doing a "go get" will install the
Elvish binary as "src.elv.sh"; this is not what we want, so move the main
package into a package called "elvish".

Also update "go get" and "go build" commands accordingly.

Makefile

@@ -3,7 +3,7 @@ default: test get
 get:
 	go get -trimpath -ldflags \
 		"-X src.elv.sh/pkg/buildinfo.Version=$$(git describe --tags --always --dirty=-dirty) \
-		 -X src.elv.sh/pkg/buildinfo.Reproducible=true" .
+		 -X src.elv.sh/pkg/buildinfo.Reproducible=true" ./cmd/elvish
 
 # Used by elves/up
 buildall:

README.md

@@ -53,7 +53,7 @@ This will install Elvish to `~/go/bin`.
 Alternatively, you can also just use `go get` to install Elvish:
 
 ```sh
-go get -u src.elv.sh
+go get -u src.elv.sh/cmd/elvish
 ```
 
 This will clone the Git repository to `~/go/src/src.elv.sh`, updating it if

tools/buildall.sh

@@ -39,7 +39,7 @@ buildone() {
     echo -n "Building for $GOOS-$GOARCH... "
     go build -o $DST_DIR/$BIN -trimpath -ldflags \
         "-X src.elv.sh/pkg/buildinfo.Version=$VERSION \
-         -X src.elv.sh/pkg/buildinfo.Reproducible=true" || {
+         -X src.elv.sh/pkg/buildinfo.Reproducible=true" ./cmd/elvish || {
         echo "Failed"
         return
     }