simplify install_tag handling according to the accepted API

There are two problems here: a typing problem, and an algorithm problem.

We expect it to always be passed to CustomTarget() as a list, but we ran
list() on it, which became horribly mangled if you violated the types
and passed a string instead. This caused weird*er* errors and didn't
even do anything. We want to do all validation in the interpreter,
anyway, and make the build level dumb.

Meanwhile we type it as accepting a T.Sequence, which technically
permits... a string, actually. This isn't intentional; the point of
using T.Sequence is out of a misguided idea that APIs are supposed to be
"technically correct" by allowing "anything that fulfills an interface",
which is a flawed concept because we aren't using interfaces here, and
also because "technically string fulfills the same interface as a list,
if we're talking sequences".

Basically:
- mypy is broken by design, because it typechecks "python", not "what we
  wish python to be"
- we do not actually need to graciously permit passing tuples instead of
  lists

As far as historic implementations of this logic go, we have formerly:
- originally, typeslistified anything
- switched to accepting list from the interpreter, redundantly ran list()
  on the list we got, and mishandling API violations passing a string
  (commit 11f96380351a88059ec55f1070fdebc1b1033117)
- switched to accepting anything, stringlistifying it if it was not
  `None`, mishandling `[None]`, and invoking list(x) on a brand new list
  from stringlistify (commit 157d43883515507f42618b065a64fb26501734a0)
- stopped stringlistify, just accept T.List[str | None] and re-cast to
  list, violates typing because we use/handle plain None too
  (commit a8521fef70ef76fb742d80aceb2e9ed634bd6a70)
- break typing by declaring we accept a simple string, which still
  results in mishandling by converting 'foo' -> ['f', 'o', 'o']
  (commit ac576530c43734495815f22456596772a8f6a8cc)

All of this. ALL of it. Is because we tried to be fancy and say we
accept T.Tuple; the only version of this logic that has ever worked
correctly is the original untyped do-all-validation-in-the-build-phase
typeslistified version.

Let's just call it what it is. We want a list | None, and we handle it too.

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