internal/getproviders: Stub out the two mirror sources

In a future commit, these implementations of Source will allow finding
and retrieving provider packages via local mirrors, both in the local
filesystem and over the network using an HTTP-based protocol.

internal/getproviders/filesystem_mirror_source.go

@@ -0,0 +1,37 @@
+package getproviders
+
+import (
+	"github.com/hashicorp/terraform/addrs"
+)
+
+// FilesystemMirrorSource is a source that reads providers and their metadata
+// from a directory prefix in the local filesystem.
+type FilesystemMirrorSource struct {
+	baseDir string
+}
+
+var _ Source = (*FilesystemMirrorSource)(nil)
+
+// NewFilesystemMirrorSource constructs and returns a new filesystem-based
+// mirror source with the given base directory.
+func NewFilesystemMirrorSource(baseDir string) *FilesystemMirrorSource {
+	return &FilesystemMirrorSource{
+		baseDir: baseDir,
+	}
+}
+
+// AvailableVersions scans the directory structure under the source's base
+// directory for locally-mirrored packages for the given provider, returning
+// a list of version numbers for the providers it found.
+func (s *FilesystemMirrorSource) AvailableVersions(provider addrs.Provider) (VersionList, error) {
+	// TODO: Implement
+	panic("FilesystemMirrorSource.AvailableVersions not yet implemented")
+}
+
+// PackageMeta checks to see if the source's base directory contains a
+// local copy of the distribution package for the given provider version on
+// the given target, and returns the metadata about it if so.
+func (s *FilesystemMirrorSource) PackageMeta(provider addrs.Provider, version Version, target Platform) (PackageMeta, error) {
+	// TODO: Implement
+	panic("FilesystemMirrorSource.PackageMeta not yet implemented")
+}

internal/getproviders/http_mirror_source.go

@@ -0,0 +1,38 @@
+package getproviders
+
+import (
+	"net/url"
+
+	"github.com/hashicorp/terraform/addrs"
+)
+
+// HTTPMirrorSource is a source that reads provider metadata from a provider
+// mirror that is accessible over the HTTP provider mirror protocol.
+type HTTPMirrorSource struct {
+	baseURL *url.URL
+}
+
+var _ Source = (*HTTPMirrorSource)(nil)
+
+// NewHTTPMirrorSource constructs and returns a new network mirror source with
+// the given base URL. The relative URL offsets defined by the HTTP mirror
+// protocol will be resolve relative to the given URL.
+func NewHTTPMirrorSource(baseURL *url.URL) *HTTPMirrorSource {
+	return &HTTPMirrorSource{
+		baseURL: baseURL,
+	}
+}
+
+// AvailableVersions retrieves the available versions for the given provider
+// from the object's underlying HTTP mirror service.
+func (s *HTTPMirrorSource) AvailableVersions(provider addrs.Provider) (VersionList, error) {
+	// TODO: Implement
+	panic("HTTPMirrorSource.AvailableVersions not yet implemented")
+}
+
+// PackageMeta retrieves metadata for the requested provider package
+// from the object's underlying HTTP mirror service.
+func (s *HTTPMirrorSource) PackageMeta(provider addrs.Provider, version Version, target Platform) (PackageMeta, error) {
+	// TODO: Implement
+	panic("HTTPMirrorSource.PackageMeta not yet implemented")
+}