Fix the --source-jar option & add corresponding using directives

[Gedochao]

Fixes #2097

It turns out the --source-jar command line option was implemented, but not at all passed to the build (and thus, non-functioning).
I fixed it, added corresponding directives and added some tests & docs.

//> using sourceJar path/to/jar-sources

Additionally, the code now assumes any JARs with the *-sources.jar name suffix are assumed to be sources JARs, and are treated appropriately (along with a warning).

scala-cli . --extra-jar '/path/to/custom-sources.jar'
# [warn] Jars with the *-sources.jar name suffix are assumed to be source jars.
# The following jars were assumed to be source jars and will be treated as such: /path/to/custom-sources.jar

[lwronski]

LGTM