when generating optional utility targets in ninja, skip existing aliases too

When auto-generating e.g. a `clang-format` target, we first check to see
if the user has already defined one, and if so we don't bother creating
our own. We check for two things:

- if a ninja target already exists, skip
- if a run_target was defined, skip

The second check is *obviously* a duplicate of the first check. But the
first check never actually worked, because all_outputs was only
generated *after* generating all utility rules and actually writing out
the build.ninja file. The check itself compares against nothing, and
always evaluates to false no matter what.

Fix this by reordering the target creation logic so we track outputs
immediately, but only error about them later. Now, we no longer need to
special-case run_target at all, so we can drop that whole logic from
build.py and interpreter.py, and simplify the tracked state.

Fixes defining an `alias_target()` for a utility, which tried to
auto-generate another rule and errored out. Also fixes doing the same
thing with a `custom_target()` although I cannot imagine why anyone
would want to produce an output file named `clang-format` (unless clang
itself decided to migrate to Meson, which would be cool but feels
unlikely).

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