This fixes Ctrl-C not interrupting cabal properly while it is downloading. I believe
that might be #6322, although it would be good to verify that.

There are two three concrete bugs:

• try catches asynchronous exceptions, preventing withAsync from interrupting it when its body action finishes prematurely.
• the use of withAsync is not safe since we don't wait for the async and hence lose exceptions it might throw
• the use of regular bracket in withTempFileName can "convert" AsyncCancelled into a regular IO exception

The first bug manifests during cabal build when hitting Ctrl-C during
download, e.g.:

1. Ctrl-C interrupts a curl process
2. This interrupt is converted into a UserInterrupt exception
   via the process package's delegate_ctlc functionality.
3. UserInterrupt bubbles up through fetchPackage, is caught by
   the try (fine) and written to the result mvar.
   Meanwhile, fetchPackage continues its loop, starting to
   fetch the next package.
4. Some rebuildTarget is waiting for the interrupted download;
   waitAsyncFetchPackage rethrows UserInterrupt in that thread.
5. UserInterrupt bubbles up through collectJob, execute, the
   body action.
6. withAsync's finalizer cancels the fetchPackages async.
7. The fetchPackages async receives the AsyncCancelled exception
   while fetching the next package (see 3. above). This interrupts
   the download action (it shouldn't matter whether we happen to
   be running curl again or not), and AsyncCancelled bubbles up
   through fetchPackage, is caught by the try (not fine!) and
   written to the mvar. But no-one is reading that mvar anymore
   because the build job already aborted (step 5), and that
   AsyncCancelled exception is lost.
8. fetchPackages keeps doing its thing, exiting eventually.

To fix this, use try from safe-exceptions, which doesn't catch
asynchronous exceptions. Note that this change affects both
instances of try in this story: UserInterrupt is also an asynchronous
exception, so after the change the UserInterrupt takes a different path
from step 3.

This relates to the second bug, uncovered during testing of the fix for
the first bug: UserInterrupt can now cause the async part of
withAsync to crash. That one is fixed by using concurrently instead
of withAsync.

Finally, the third bug showed on Windows CI during testing, where temporary
file cleanup trigged by async cancellation fails with an IO exception, which is
then treated as a regular download failure, while the attempt to cancel the
async is ignored.

• To verify manually, call cabal build for a package with several undownloaded dependencies, and hit Ctrl-C when it gets started. Prior to these changes, cabal doesn't exit until it's done downloading, afterwards it does. I've checked this on macOS.
• The PR includes unit tests for asyncFetchPackages with a faked HTTP transport, simulating several failure scenarios where the asyncFetchPackages might not exit promptly or even deadlock. Some of these fail before the change, and now pass.

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.