install.sh: Add error for trying to install two compilers

[dlangBugzillaToGithub]

John Hall reported this on 2021-03-31T23:43:33Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=21786

Description

A command like

~/dlang/install.sh install ldc,dub

installs ldc and a corresponding version of dub. However, install.sh does not support commands here other than dub. For instance, a call like

~/dlang/install.sh install ldc,dmd

will return errors like

Downloading and unpacking https://github.com/dlang/dub/releases/download/v/dub-v-linux-x86_64.tar.gz
curl: (22) The requested URL returned error: 404  

Instead, there should be a check that anything after the comma starts with dub and have an error if it is not. 

Alternately, it could loop through anything comma'ed so that the above command installs both ldc and dmd.