Resolve sources jars for the whole classpath, per entry

[rochala]

Summary

• `get-source` only fetched the sources jar of the coordinate the user named, so a symbol defined in a dependency (inherited members, `cats.Functor` via cats-effect) could not be shown.
• The `sources` classifier is now resolved in the same coursier fetch as the binaries. Coursier treats classifier artifacts as optional, so a dependency without a sources jar simply has none (verified with a local artifact lacking one).
• `ContextResource.makeFromCoord` yields a `SourceJars` map from tasty-query classpath entry to sources jar; `get-source` looks the jar up via the symbol's own entry.
• Removes the now-unused `fetchSourcesJar`; README row for `get-source` updated.

Verified

get-source org.typelevel:cats-effect_3:3.5.4 cats.Functor      → cats/Functor.scala lines 31–272  (failed before)
get-source org.typelevel:cats-effect_3:3.5.4 cats.effect.IOApp → cats/effect/IOApp.scala lines 143–594
get-source org.scalameta:mtags-interfaces:1.3.5 scala.meta.pc.OffsetParams → scala/meta/pc/OffsetParams.java

Full `lib.test` passes; `ContextResourceTest` asserts the fixture maps to its own sources jar and `scala.Option` to scala-library's. `_.fix --check` clean.

Not included

JDK sources (`lib/src.zip`) and project-aware commands, where the build tool gives jar paths without coordinates. First run of any external command now also downloads the closure's sources jars (cached afterwards).

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