treewide: internally avoid deprecated machine file uses of "pkgconfig"

We support this in a machine file:

```
[binaries]
pkgconfig = 'pkg-config'
pkg-config = 'pkg-config'
```

and you can use either one, because internally we look up both. If you
only set *one* of them, this plays awkwardly with setting $PKG_CONFIG,
since we don't know which one you set in the machine file and the
*other* one will be initialized from the environment instead.

In commit 22df45a31981874310a78dde0df59a6a7c5ebb29 we changed program
lookup of config-tool style dependencies to use the regular tool names
and only fall back on the strange internal names. This affected the
pkg-config class too.

The result is that instead of preferring `pkgconfig =` followed by
$PKG_CONFIG followed by `pkg-config =`, we inverted the lookup order.
This is a good idea anyway, because now it behaves consistently with
`find_program('pkg-config')`.

Unfortunately, we documented the wrong name in a bunch of places, and
also used the wrong name in various testsuite bits, which meant that if
you set $PKG_CONFIG and then ran the testsuite, it would fail.

Correct these references, because they are buggy.

One test case expected to find_program() a native copy for convenience
of testing against natively installed glib. Force it to resolve a native
copy.

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