Allow nvm-exec to be linked into individual .nvm directories for system-wide installs with a localized Nodes.

Let's say we have nvm installed in a separate mount, /.socket. NVM_DIR is $HOME/.nvm in /etc/profile.d/nvm.sh. With this setup, users can install Node versions to their home directories without each installing nvm.

nvm install --lts

This works fine as does nvm use --lts. When nvm exec is used though, it fails because it looks for nvm-exec in $NVM_DIR. First fix is to look for nvm-exec in $NVM_DIR. If NVM_DIR does not contain nvm-exec, check $BASH_SOURCE[0]. The second fix is to follow nvm-exec if a symbolic link to determine the proper location of nvm's home. Alternatively we could use a second environment variable, NVM_HOME in exec instead of relying on the directory name of nvm-exec.

nvm-exec

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 
-DIR="$(command cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+SOURCE=${BASH_SOURCE[0]}
+test -L "$SOURCE" && SOURCE=`readlink "$SOURCE"`
+DIR="$(command cd "$( dirname "$SOURCE" )" && pwd )"
 
 # shellcheck disable=SC1090
 \. "$DIR/nvm.sh" --no-use

nvm.sh

@@ -3201,7 +3201,9 @@ nvm() {
           nvm_echo "Running node $VERSION$(nvm use --silent "$VERSION" && nvm_print_npm_version)"
         fi
       fi
-      NODE_VERSION="$VERSION" "$NVM_DIR/nvm-exec" "$@"
+      NVM_EXEC="$NVM_DIR/nvm-exec"
+      test ! -f "$NVM_EXEC" && NVM_EXEC=`dirname ${BASH_SOURCE[0]-}`/nvm-exec
+      NODE_VERSION="$VERSION" "$NVM_EXEC" "$@"
     ;;
     "ls" | "list" )
       local PATTERN