Support load("uv.lock?as=toml", "value") syntax

[cormacrelf]

Alternative to #1286.

The use case is reading files at bzl eval time that don't have a .toml suffix, as TOML, for https://github.com/cormacrelf/elk which I have updated to depend on this entire thing at the heart of its strategy only to realise it doesn't work very well at all. (I would be happy with a way to tell load() to load as TOML if we can't get symlinks to work.)

Here's that alternative. The example becomes

# BUCK
load(":blah.lock?as=toml", "value")

genrule(
    name = "test",
    out = "out.txt",
    cmd = "echo {} > $OUT".format(value),
)

# blah.lock
value = "original"

and basically this works. This code I do understand quite well, and it's simple and there are no X factors as to how to get it to work.

[meta-codesync]

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