dependencies: Use CompilerType not Compiler

Compiler is invariant, in other words Compiler and only Compiler can
fulfull it, it's derived classes cannot be used. CompilerType is
covariant, that is Compiler and any derived class can fulfill it. This
fixes a number of issues in the boost module.

mesonbuild/dependencies/base.py

@@ -38,7 +38,7 @@ from ..mesonlib import Popen_safe, version_compare_many, version_compare, listif
 from ..mesonlib import Version, LibType
 
 if T.TYPE_CHECKING:
-    from ..compilers.compilers import Compiler  # noqa: F401
+    from ..compilers.compilers import CompilerType  # noqa: F401
     DependencyType = T.TypeVar('DependencyType', bound='Dependency')
 
 # These must be defined in this file to avoid cyclical references.
@@ -2488,7 +2488,7 @@ def factory_methods(methods: T.Set[DependencyMethods]) -> 'FactoryType':
 
 
 def detect_compiler(name: str, env: Environment, for_machine: MachineChoice,
-                    language: T.Optional[str]) -> T.Optional['Compiler']:
+                    language: T.Optional[str]) -> T.Optional['CompilerType']:
     """Given a language and environment find the compiler used."""
     compilers = env.coredata.compilers[for_machine]