Remove Configure code that supported the old-style nested layout for ext/

5.10.1 switched to the new layout, so this code would only be useful for
backporting to maint-5.8. That isn't going happen, so it can go.
Add non-XS extensions to known_extensions.

Previously "known_extensions" was misnamed, as it only contained known XS
extensions. grep.cpan.me suggests that there are only 10 mentions of it
outside the core, and none of them rely on this existing behaviour.

These changes need replicating into configure.com.
In FindExt, eliminate _ext_ne() and make extensions() a simple subroutine.

Previously _ext_ne() was a generator function, and extensions() and
known_extensions() were generated by it. Now that known_extensions() has a
different implementation, extensions() was the last user of _ext_ne(), so
there's no saving by keeping the complexity.
Correct a type in FindExt::apply_config in the "I18N-Langinfo" code.

Commit 557ab4cb986767c7 (Feb 2011) which added this routine had
$config->{i_nl_langinfo} not $config->{d_nl_langinfo} ('i' should be 'd').

The logic for I18N::Langinfo should now be correct.
Thanks to FindExt::apply_config() we're now able to test dynamic extensions.

FindExt::apply_config() mimic's Configure's logic, which means that FindExt's
idea of which extensions should be built is consistent with Configure's.
Minor refactors to FindExt's test, removing code duplication.

Use a ternary instead of if/unless on the same $^O test.
Use a loop to call FindExt::scan_ext()
As FindExt exports nothing, we can require it rather than using it.
In FindExt.t, move the main loop's comparison logic into a subroutine.
Skip most of FindExt's tests for troublesome configurations.

There are various various things that break the test's assumptions.
1) If Encode is a static extension, then Configure has special case logic
   to add Encode/* as static extensions
2) -Uusedl causes Encode to be a static extension, and drops building
    XS::APItest and XS::Typemap
3) Any use of -Dnoextensions to choose not to build a extension is not known
   by the test

If any of these are true, FindExt::extensions() and $Config{extensions} will
differ, and most of the tests are going to fail. Moreover, failure doesn't
tell us anything interesting. So don't run those tests.
Refactor FindExt, merging scan_ext() and find_ext().

The return value of FindExt::scan_ext() has never been used, since FindExt
was first added by commit 8e2329934bcca9c5 in April 2001. The call to
FindExt::extensions() has no side effects, so it can be eliminated. Hence
FindExt::scan_ext() is a trivial wrapper around FindExt::find_ext(), and the
two can be merged.

Also, simplify the logic for "known" extensions. The complexity of checking
the hash first was needed when extension directories were nested. It should
have been removed as part of commit 1f8a0b38638b171c in Feb 2009.
vms/ext/filespec.t does not need to be +x

The mode of this file has rattled back and forth between +x and -x since it
was first added. It makes no difference which it is, so remove -x and hence
1 special case.
Move VMS::Filespec from vms/ext to ext/

This simplifies the VMS Makefile. It would have simplified the VMS Makefile
further if it had had the correct rules to delete [.lib.VMS]Filespec.pm
which are now no longer needed. (The generated ext/VMS-Filespec/DESCRIP.MMS
will now take care of this.)
Remove defunct DESCRIP.MMS cleanup rules.

Rules to remove C and object files from vms/ext were made redundant when
commit 26dd53a231877708 in Sep 2009 moved the XS extensions from there to
ext/

The wildcard rule to remove t/lib/vms*.t, which has been in
vms/descrip_mms.template since the file was added by commit 97abc6adffcd3efc
in June 1998 was effectively made redundant when it was duplicated by 4
specific rules for the 4 files it matched added by commit 493ba88a837f5a6b
in June 2001.

The rule to delete t/lib/vmsish.t was made redundant when vms/ext/vmsish.*
were moved to lib/ by commit 9f84c00564fd021b in Nov 2001.