SI-6502 Reenables loading jars into the running REPL (regression in 2.10)

Fixes [SI-6502](https://issues.scala-lang.org/browse/SI-6502), reenables loading jars into the running REPL (regression in 2.10). This PR allows adding a jar to the compile and runtime classpaths without resetting the REPL state (crucial for Spark [SPARK-3257](issues.apache.org/jira/browse/SPARK-3257)).

This follows the lead taken by @som-snytt in PR [#3986](https://github.com/scala/scala/pull/3986), which differentiates two jar-loading behaviors (muddled by `cp`):
 - adding jars and replaying REPL expressions (using `replay`)
 - adding jars without resetting the REPL (deprecated `cp`, introduced `require`)
This PR implements `require` (left unimplemented in [#3986](https://github.com/scala/scala/pull/3986))

This PR is a simplification of a similar approach taken by @gkossakowski in #3884. In this attempt, we check first to make sure that a jar is only added if it only contains _new_ classes/traits/objects, otherwise we emit an error. This differs from the old invalidation approach which also tracked deleted classpath entries.
This conditional seems awkwardly arranged.

How about:

```
if (!f.exists) echo("The path '" + f + "' doesn't seem to exist.")
else if (exists) echo("The path '" + f + "' cannot be loaded, because existing classpath entries conflict.") // TODO tell me which one
else ...