Allow attaching sources to dependencies

[cursive-ide]

Often, especially when working with e.g. Java dependencies, it's useful to be able to attach sources to the libraries that are created when a project is imported. IntelliJ does this by default for all Maven or Gradle dependencies, but this is very slow when using Clojure since most artefacts are published without source (or, more correctly, the sources are actually the binary artefact). If Cursive tries to download sources for all artefacts, it is very slow since the Maven resolver will go out to all the remote repositories for every artefact looking for sources which will never exist.

Maven-like things publish the source artefacts using the "sources" classifier. In deps, these can be downloaded using, e.g.:

junit/junit {:mvn/version "4.13.2"}
junit/junit$sources {:mvn/version "4.13.2"}

Cursive should detect if a downloaded artefact contains sources for another, and attach it if so. The sources could also be specified in an alias which is used to sync to Cursive, meaning that the sources will not be resolved for other uses, e.g:

{:deps    {junit/junit {:mvn/version "4.13.2"}}
 :aliases {:ide {:extra-deps {junit/junit$sources {:mvn/version "4.13.2"}}}}}

This will allow the user to specify which dependencies they want sources downloaded for in a non-intrusive way.