python: Fix using library from PEP 739 without pkg-config

Fix the `find_libpy()` PEP 739 branch to handle the full path correctly.
Meson's `find_library()` function accepts only the "bare" name, while
`build-details.json` provides the full path to the library. The previous
code would just extract the basename, but it would still contain
the library prefix and suffix. Rather than trying to hack around that,
just use the full path directly.

Author: mgorny

mesonbuild/dependencies/python.py

@@ -295,23 +295,26 @@ def find_libpy(self, environment: 'Environment') -> None:
             path = self.build_config['libpython'].get('dynamic')
             if not path:
                 raise DependencyException('Python does not provide a dynamic libpython library')
-            libdirs = [os.path.dirname(path)]
-            libname = os.path.basename(path)
-        else:
-            if self.is_pypy:
-                if self.major_version == 3:
-                    libname = 'pypy3-c'
-                else:
-                    libname = 'pypy-c'
-                libdir = os.path.join(self.variables.get('base'), 'bin')
-                libdirs = [libdir]
+            if not os.path.isfile(path):
+                raise DependencyException('Python dynamic library does not exist or is not a file')
+            self.link_args = [path]
+            self.is_found = True
+            return
+
+        if self.is_pypy:
+            if self.major_version == 3:
+                libname = 'pypy3-c'
             else:
-                libname = f'python{self.version}'
-                if 'DEBUG_EXT' in self.variables:
-                    libname += self.variables['DEBUG_EXT']
-                if 'ABIFLAGS' in self.variables:
-                    libname += self.variables['ABIFLAGS']
-                libdirs = []
+                libname = 'pypy-c'
+            libdir = os.path.join(self.variables.get('base'), 'bin')
+            libdirs = [libdir]
+        else:
+            libname = f'python{self.version}'
+            if 'DEBUG_EXT' in self.variables:
+                libname += self.variables['DEBUG_EXT']
+            if 'ABIFLAGS' in self.variables:
+                libname += self.variables['ABIFLAGS']
+            libdirs = []
 
         largs = self.clib_compiler.find_library(libname, environment, libdirs)
         if largs is not None: