Installing LSPHP under Ubuntu will install Ubuntu's default PHP dependencies

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OS:Ubuntu 18.04

When I installed LSPHP73,
E.g:
```bash
root @ ubuntu-s-1vcpu-1gb-sfo2-01: ~ # apt install lsphp73
Reading package lists ... Done
Building dependency tree
Reading state information ... Done
The following package was automatically installed and is no longer required:
  grub-pc-bin
Use 'apt autoremove' to remove it.
The following additional packages will be installed:
  aspell aspell-en dictionaries-common emacsen-common enchant hunspell-en-us libaspell15 libenchant1c2a libhunspell-1.6-0 libjpeg-turbo8 libjpeg8 libsodium23 libxpm4 libzip4 lsphp73-common lsphp73-json lsphp73-opcache php-common
  php-readline php7.2-common php7.2-readline
Suggested packages:
  aspell-doc spellutils wordlist hunspell openoffice.org-hunspell | openoffice.org-core libenchant-voikko php-pear php-user-cache
The following NEW packages will be installed:
  aspell aspell-en dictionaries-common emacsen-common enchant hunspell-en-us libaspell15 libenchant1c2a libhunspell-1.6-0 libjpeg-turbo8 libjpeg8 libsodium23 libxpm4 libzip4 lsphp73 lsphp73-common lsphp73-json lsphp73-opcache php-common
  php-readline php7.2-common php7.2-readline
0 upgraded, 22 newly installed, 0 to remove and 73 not upgraded.
Need to get 11.6 MB of archives.
After this operation, 58.9 MB of additional disk space will be used.
```
Will install php-common php-readline, these two unreliable dependencies. At the same time, they will bring more ubuntu default php extension dependencies, which will complicate the environment. Under docker, the mirror will also become very Big.

Start by reproducing `apt install lsphp73` on Ubuntu 18.04 and inspect the package dependency metadata that pulls in `php-common`, `php-readline`, and PHP 7.2 packages. Done means LSPHP73 installs without the unwanted Ubuntu PHP dependencies and the Docker image no longer gains those packages.

Written by the indexing model from the issue text.