dependencies: delay often-unused imports

We expose detect.py as the mesonbuild.dependencies entrypoint and import
it upfront everywhere. But unless the `dependency()` function is
actually invoked, we don't need *any* of the private implementations for
this.

Avoid doing so until, as part of actual dependency lookup, we attempt
that specific dependency method. This avoids importing big modules if
`method:` is specified, and in most cases hopefully pkg-config works and
we can avoid importing the cmake implementation particularly.

Actually avoiding most of these imports requires more refactoring. But
even so, the garden path no longer needs to import the dub dependency
impl.

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