* docs: clarify write_source_files output constraint

For both the "write_source_files" rule's "files" attribute and the
"write_source_file" rule's "out_file" attribute, we ensure that the
destination files and directories to which we'll write sit within the
same containing Bazel package as the Bazel target requesting this
writing. Clarify the documentation for each attribute to make it clear
that we're constraining the destinations and not the sources of the
content to be written.

* Update lib/write_source_files.bzl

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Co-authored-by: Derek Cormier <derek@aspect.dev>

…504)

Our Engflow cluster seems to be down.

It causes warnings like https://github.com/bazelbuild/bazel/blob/753cd459e49d1ffae1420bf5d46db228449299c6/src/main/java/com/google/devtools/build/lib/sandbox/SandboxModule.java#L514
and also makes toolchain selection broken for host!=exec

Fixes #466

On windows, we can't turn on the Aspect CLI currently as we don't have a release

* yq: allow any filename in srcs

* yq: add test cases for free-form file extensions in input

* run buildifier

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Co-authored-by: Alex Eagle <alex@aspect.dev>