env: validate PEP 508 specifiers before passing them as arguments to pip

Per the specification for pyproject.toml build-system.requires, these
MUST be valid PEP 508 strings, and when checking to see if they are
satisfied during --no-isolation builds, they will be parsed using
`packaging.requirements` which checks this for free as a side effect of
converting each string into a good format for checking against
importlib.metadata dists.

But when building by default, they are just pip installed and pip
supports many things that aren't just PEP 508 strings. Fix this by
first parsing them with `packaging.requirements` just to get the error
message.

src/build/env.py

@@ -115,13 +115,16 @@ def install(self, requirements: Collection[str]) -> None:
         Install packages from PEP 508 requirements in the isolated build environment.
 
         :param requirements: PEP 508 requirement specification to install
-
-        :note: Passing non-PEP 508 strings will result in undefined behavior, you *should not* rely on it. It is
-               merely an implementation detail, it may change any time without warning.
         """
         if not requirements:
             return
 
+        import packaging.requirements
+
+        for r in requirements:
+            # raise exception for invalid requirements
+            packaging.requirements.Requirement(r)
+
         self.log(f'Installing packages in isolated environment... ({", ".join(sorted(requirements))})')
 
         # pip does not honour environment markers in command line arguments