Add aup watch: build again when what a build read changes

A build records every path it opened or looked for and a digest of the
bytes there, so a rebuild happens exactly when one of them reads
differently: the source, a library (the one with the error too), or a
library appearing where one was searched. A change is built once it reads
the same twice in a row, so a file caught halfway through being written
is never built; the module is replaced whole through a rename.

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