fix broken type annotation imports being ignored

If an annotation could not be resolved, it's classified as a "missing
import" and our configuration ignored it:

```
Skipping analyzing "mesonbuild.backends": module is installed, but missing library stubs or py.typed marker
```

As far as mypy is concerned, this library may or may not exist, but it
doesn't have any typing information at all (may need to be installed
first).

We ignored this because of our docs/ and tools/ thirdparty dependencies,
but we really should not. It is trivial to install them, and then
enforce that this "just works".

By enforcing it, we also make sure typos get caught.

6 files changed, 10 insertions, 8 deletions

diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index 547a520cc..ddedabf7a 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -46,7 +46,7 @@ jobs:
     - uses: actions/setup-python@v4
       with:
         python-version: '3.x'
-    - run: python -m pip install mypy types-PyYAML
+    - run: python -m pip install mypy coverage types-PyYAML types-tqdm types-chevron
     - run: python run_mypy.py --allver
       env:
         PYTHONUNBUFFERED: 1
diff --git a/.mypy.ini b/.mypy.ini
index 70fdcd9f3..d00944bb2 100644
--- a/.mypy.ini
+++ b/.mypy.ini
@@ -2,7 +2,7 @@
 strict_optional               = False
 show_error_context            = False
 show_column_numbers           = True
-ignore_missing_imports        = True
+ignore_missing_imports        = False
 implicit_reexport             = False
 
 follow_imports                = silent
diff --git a/docs/refman/loaderyaml.py b/docs/refman/loaderyaml.py
index fc06b0d67..e7ec57a7e 100644
--- a/docs/refman/loaderyaml.py
+++ b/docs/refman/loaderyaml.py
@@ -41,7 +41,7 @@ class Template:
 
 class StrictTemplate(Template):
     def __init__(self) -> None:
-        from strictyaml import Map, MapPattern, Optional, Str, Seq, Int, Bool, EmptyList, OrValidator
+        from strictyaml import Map, MapPattern, Optional, Str, Seq, Int, Bool, EmptyList, OrValidator # type: ignore[import-untyped]
 
         d_named_object = {
             'name': Str(),
diff --git a/mesonbuild/compilers/mixins/compcert.py b/mesonbuild/compilers/mixins/compcert.py
index ac4d5aaa0..a40f635fa 100644
--- a/mesonbuild/compilers/mixins/compcert.py
+++ b/mesonbuild/compilers/mixins/compcert.py
@@ -20,7 +20,7 @@ import re
 import typing as T
 
 if T.TYPE_CHECKING:
-    from envconfig import MachineInfo
+    from ...envconfig import MachineInfo
     from ...environment import Environment
     from ...compilers.compilers import Compiler
 else:
diff --git a/mesonbuild/interpreter/compiler.py b/mesonbuild/interpreter/compiler.py
index 5528abe7a..ad6eb4b89 100644
--- a/mesonbuild/interpreter/compiler.py
+++ b/mesonbuild/interpreter/compiler.py
@@ -30,7 +30,7 @@ if T.TYPE_CHECKING:
     from ..compilers import Compiler, RunResult
     from ..interpreterbase import TYPE_var, TYPE_kwargs
     from .kwargs import ExtractRequired, ExtractSearchDirs
-    from .interpreter.interpreter import SourceOutputs
+    from .interpreter import SourceOutputs
     from ..mlog import TV_LoggableList
 
     from typing_extensions import TypedDict, Literal
@@ -856,7 +856,8 @@ class CompilerHolder(ObjectHolder['Compiler']):
     )
     def preprocess_method(self, args: T.Tuple[T.List['mesonlib.FileOrString']], kwargs: 'PreprocessKW') -> T.List[build.CustomTargetIndex]:
         compiler = self.compiler.get_preprocessor()
-        sources: 'SourceOutputs' = self.interpreter.source_strings_to_files(args[0])
+        _sources: T.List[mesonlib.File] = self.interpreter.source_strings_to_files(args[0])
+        sources = T.cast('T.List[SourceOutputs]', _sources)
         if any(isinstance(s, (build.CustomTarget, build.CustomTargetIndex, build.GeneratedList)) for s in sources):
             FeatureNew.single_use('compiler.preprocess with generated sources', '1.1.0', self.subproject,
                                   location=self.current_node)
diff --git a/mesonbuild/mdevenv.py b/mesonbuild/mdevenv.py
index 9f3d1b973..c8d0144c5 100644
--- a/mesonbuild/mdevenv.py
+++ b/mesonbuild/mdevenv.py
@@ -5,6 +5,7 @@ import argparse
 import tempfile
 import shutil
 import itertools
+import typing as T
 
 from pathlib import Path
 from . import build, minstall
@@ -12,9 +13,9 @@ from .mesonlib import (EnvironmentVariables, MesonException, is_windows, setup_v
                        get_wine_shortpath, MachineChoice)
 from . import mlog
 
-import typing as T
+
 if T.TYPE_CHECKING:
-    from .backends import InstallData
+    from .backend.backends import InstallData
 
 POWERSHELL_EXES = {'pwsh.exe', 'powershell.exe'}