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 11f9638035)
- 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 157d438835)
- stopped stringlistify, just accept T.List[str | None] and re-cast to
  list, violates typing because we use/handle plain None too
  (commit a8521fef70)
- break typing by declaring we accept a simple string, which still
  results in mishandling by converting 'foo' -> ['f', 'o', 'o']
  (commit ac576530c4)

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.

mesonbuild/build.py

@@ -117,15 +117,15 @@ known_shmod_kwargs = known_build_target_kwargs | {'vs_module_defs'}
 known_stlib_kwargs = known_build_target_kwargs | {'pic', 'prelink'}
 known_jar_kwargs = known_exe_kwargs | {'main_class', 'java_resources'}
 
-def _process_install_tag(install_tag: T.Optional[T.Sequence[T.Optional[str]]],
+def _process_install_tag(install_tag: T.Optional[T.List[T.Optional[str]]],
                          num_outputs: int) -> T.List[T.Optional[str]]:
     _install_tag: T.List[T.Optional[str]]
     if not install_tag:
         _install_tag = [None] * num_outputs
     elif len(install_tag) == 1:
-        _install_tag = list(install_tag) * num_outputs
+        _install_tag = install_tag * num_outputs
     else:
-        _install_tag = list(install_tag)
+        _install_tag = install_tag
     return _install_tag
 
 
@@ -2418,7 +2418,7 @@ class CustomTarget(Target, CommandBase):
                  install: bool = False,
                  install_dir: T.Optional[T.Sequence[T.Union[str, Literal[False]]]] = None,
                  install_mode: T.Optional[FileMode] = None,
-                 install_tag: T.Optional[T.Sequence[T.Optional[str]]] = None,
+                 install_tag: T.Optional[T.List[T.Optional[str]]] = None,
                  absolute_paths: bool = False,
                  backend: T.Optional['Backend'] = None,
                  ):

mesonbuild/modules/fs.py

@@ -304,7 +304,7 @@ class FSModule(ExtensionModule):
             install=kwargs['install'],
             install_dir=kwargs['install_dir'],
             install_mode=kwargs['install_mode'],
-            install_tag=kwargs['install_tag'],
+            install_tag=[kwargs['install_tag']],
             backend=state.backend,
         )