Install .dmg package on MacOS Catalina.

Example with Vagrant package.

* Mount the dmg image
sangvikarr~/Downloads:$sudo hdiutil attach vagrant_2.2.6_x86_64.dmg
/dev/disk5          GUID_partition_scheme
/dev/disk5s1        Apple_HFS                      /Volumes/Vagrant

* Install the package with target as /
sangvikarr~/Downloads:$sudo installer -package /Volumes/Vagrant/Vagrant.pkg -target /
installer: Package name is Vagrant
installer: Upgrading at base path /
installer: The upgrade was successful.

* Finally detach the image.
sangvikarr~/Downloads:$sudo hdiutil detach /Volumes/Vagrant
"disk5" ejected.

* sangvikarr~# vagrant version
Installed Version: 2.2.6
Latest Version: 2.2.6

You're running an up-to-date version of Vagrant!

sangvikarr~# cd
sangvikarr# which vagrant
/usr/local/bin/vagrant