[namespace] Infer namespaces from filenames instead of reading ns forms

[alexander-yakushev]

This PR simplifies how finding namespaces on classpath is performed.

1. Instead of reading every .clj file on the classpath and looking for ns form in it, infer the namespace name from the filename. This is what Compliment does; and it always served well enough. Surely, this may lead to some false positives that we try to filter out to the best of our ability, but overall it's not the end of the world.
2. With file reading gone, pmap stuff is no longer necessary, the computation is fast enough serially.
3. I moved some of the file-locating code into orchard.util.classpath where it sits more cosily.
4. Removed a few more public functions, except for orchard.util.classpath/claspath-seq which is occasionally used in other projects.

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• You've updated the changelog (if adding/changing user-visible functionality)

[alexander-yakushev]

Tests pass now.