Force use index

[andrius-puksta-sensmetry]

Sysand currently allows overriding the index used to resolve packages. This does not change the resolution logic, which goes as follows:

sysand/core/src/resolve/combined.rs

Lines 18 to 42 in 689b6da

	/// Implements "standard" resolution logic given a set of individual resolvers.
	/// Use sysand::resolve::null::NullResolver to skip any of the steps.
	/// The logic is as follows:
	/// 1. Do not resolve any further if file_resolver is successful, otherwise go to step 2.
	/// 2. If remote_resolver produces any results, discard any that do not point to a valid
	/// project (i.e. do not produce both a info and meta). If at least one project is found
	/// proceed to 4. (skipping 3.)
	/// 3. Take whatever results are produced by remote_resolver and proceed to step 4.
	/// 4. If local_resolver resolves anything, collect all the results. Iterate over the results
	/// from previous steps, but interleave results from local_resolver when they have
	/// identical hashes. Any results from local_resolver that were not interleaved are returned
	/// at the end.
	///
	/// Cached values are returned exactly once (so if the underlying resolver gives duplicates
	/// they will appear cached only one time).
	///
	/// The above procedure basically amounts to:
	/// - file_resolver represents private projects
	/// - remote_resolver is prioritised, but may be ignored if it does not resolve valid projects
	/// (typically due to using non-resolving URLs to reference a resource)
	/// - registry_resolver is what will typically be hit when using non-resolvable IRIs
	/// - local_resolver serves to provide a cache, but may contain "dangling" cached projects
	///
	/// Each resolver is optional, and can be skipped by passing `None`. `NO_RESOLVER` is a typed `None`
	/// value that can be used to avoid ambiguous typing.

In case one desires to resolve all packages through a custom index (such as an exposed local sysand_env), there is no way to force the resolution to use the specified index only or to check it first. Control over where the packages are fetched from is useful in a restricted environment.

Options:

• leave it as-is and document how the package resolution works, i.e. in which order the sources are checked.
  Also mention it in docstring for --use-index, currently it's not clear that overriding the index does not force its usage or make it the primary resolver.
• add another flag --force-index (to use only the specified index) and/or --index-first (first try the specified index). Exact naming is up for debate.

[tilowiklundSensmetry]

The KerML spec says that if a usage URL does resolve, then it must resolve to a kpar (archive). Thus far, we've interpreted this as saying that (resolvable) URL should always be treated as direct links to a specific interchange project.

[andrius-puksta-sensmetry]

From the spec:

If the IRI is dereferenceable, it should resolve to a project interchange file for the used project.

Thus far, we've interpreted this as saying that (resolvable) URL should always be treated as direct links to a specific interchange project.

In my view this only places a requirement on IRI producers, not consumers. Consumers can choose to resolve the IRI to a kpar or use any other method to obtain the kpar.