…subresource() method

This makes .subresources() work as designed (though no behaviour change
is expected).  This change was missed in "sources: Refactor subresource
construction into Resource base class" (ef13cd6), which introduced the
static .Subresource property.

This reverts commit d467229.

I'm in this canonicalization code again for a new source and want to
address the query param handling centrally (rather than proliferate it
further) as mentioned in my original review.¹  Doing so includes going
back to a cleaner pathBuilder interface for routes that don't need
"req".

¹ <#793 (review)>

The same async wrapper is applied to the Express application instance,
Router instances, and Route instances (i.e. from .route()).  The latter
don't have .use().

Breaks much earlier at runtime (e.g. when setting up routes), crashing
the server, rather than allowing the server to start and erroring on
requests to the affected route.

We use .all() to avoid repeating middleware for each HTTP method on a
route.  I need to start using async middleware in those places.

Useful to be flexible here so the callback can be written less
awkwardly, and necessary to fix a bug (coming soon).

…rent Source

Fixes a subtle bug where changing the dataset *in Auspice* to a
non-canonical path caused a redirection that switched sources, e.g.
staging → core.

For example, if you were on /staging/measles/genome and switched measles
to enterovirus, Auspice made a /charon/getDataset request for
/staging/enterovirus which *should* have canonicalized to
/staging/enterovirus/d68/genome but *was* canonicalized to
/enterovirus/d68/genome.  This was demonstrable by:

    $ curl -IL https://nextstrain.org/charon/getDataset?prefix=/staging/enterovirus -so /dev/null -w '%{url_effective}\n'
    https://nextstrain.org/charon/getDataset?prefix=enterovirus%2Fd68%2Fgenome

Note that the "prefix" query param was missing "staging/".

So we can more easily do external lookups for canonicalization.

So we can more easily construct or lookup the value based on external
information.

I didn't bubble this up to Resource.baseName because it was not
necessary for my purposes, but I wouldn't be surprised if we find a need
to do that in the future.

Missed in "Derive inventory path from key in manifest" (4b2257c).

I will use this in new code that's forthcoming.

…not name

This bug had no visible effect on "core" datasets where the URL path
(e.g. /a/b/c) and name (a/b/c) are equivalent.  For other sources,
however, like "staging", the URL path (e.g. /staging/a/b/c) and name
(a/b/c) differ.  The bug remained latent because historical versions
were only ever enabled for the "core" source.  I noticed when adding UI
for a "nextclade" source that also enabled versions.

…) construction

Parameterize it so that callers can lump resources in the UI by names
other than the first slash-delimited part of the resource name.  I'll be
using this shortly.

…pName"

I'll be using this shortly to statically sort one set of resource groups
after another.