Is there any plan in the Swift PM team to add an install command to the Swift Package Manager? I think it would be a really useful feature
Executables
A first version could support executable products only. So with:
$ swift install https://url.to/git/repo
# or
$ swift install # installs the current package
Swift PM would compile all the Package executable products in release mode and install them in a known location (eg. ~/.swiftpm/bin/).
Libraries
Supporting installed libraries could be a bit more complicated as it would require deciding how to handle the Package dependencies but I think it would be very useful. For example, right now the only quick/easy-way to try a Swift PM library is to use swift run --repl (but that way you can only access one package per REPL session). Installing library products would mean that you could use all of them in a REPL session or in a Jupyter Notebook.
This is something I also want and have been casually working on a formal proposal. Unfortunately there are some complications as you pointed out with libraries that have to be resolved before an install command can be a reality. Swift 5 will hopefully open up the door to this being a reality thanks to ABI stability though.
The main questions that need answering that I am aware of are:
Where do we install libraries and executables?
How do we handle multiple versions of libraries or executables?
How are resources going to work? There are others working on a proposal for this.
Since these are the pre-requisits that I see I have been starting to figure out how to handle the libraries. Unfortunately I haven't had much time to do so the last month...