relax target name restrictions to cater to internal use

We don't want to allow targets that conflict with:
- our aliased meson-* targets for phony commands
- any meson-*/ directories we create for internal purposes

We do want to allow targets such as:
- our own meson-*.X manpages

There are a couple routes we could take.

Using a better restriction, such as `meson-internal__*`, is trivially
done for our aliased targets, but changing directory names is...
awkward. We probably cannot do this, and doing the former but not the
latter is not very useful.

We could also carefully allow patterns we know we won't use, such as
file extensions, but which the manpages need, which works for our
directories and for many aliased targets, but run_target() is
user-specified and can be anything.

Use a hybrid approach to cover both use cases. We will now allow target
names that fulfill *all* the following criteria:
- it begins with "meson-"
- it doesn't continue with "internal__"
- it has a file extension

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