zclparse: new package for the primary parser API

Although the actual parsing code will be split across multiple packages,
it's convenient for the public interface to be in a single package. The
parsers are accessed through a "Parser" type, which both wraps the
real parsers and maintains a registry of parsed files to ensure that
the same file isn't parsed more than once (which might be confusing if
there are different results) and to provide raw source code for
diagnostic-printing functions.

The parser API is in a separate package both due to the fundamental
problem that it would create an import cycle if included in the main
package and also because it's expected that calling applications will
deal with parsing once in a central place (a single file depending on
zclparse) but the resulting objects will be used broadly across the
application, causing many files to depend on the main zcl package.

zclparse/parser.go

@@ -0,0 +1,67 @@
+package parser
+
+import (
+	"github.com/apparentlymart/go-zcl/zcl"
+	"github.com/apparentlymart/go-zcl/zcl/json"
+)
+
+// NOTE: This is the public interface for parsing. The actual parsers are
+// in other packages alongside this one, with this package just wrapping them
+// to provide a unified interface for the caller across all supported formats.
+
+// Parser is the main interface for parsing configuration files. As well as
+// parsing files, a parser also retains a registry of all of the files it
+// has parsed so that multiple attempts to parse the same file will return
+// the same object and so the collected files can be used when printing
+// diagnostics.
+//
+// Any diagnostics for parsing a file are only returned once on the first
+// call to parse that file. Callers are expected to collect up diagnostics
+// and present them together, so returning diagnostics for the same file
+// multiple times would create a confusing result.
+type Parser struct {
+	files map[string]*zcl.File
+}
+
+// ParseJSON parses the given JSON buffer (which is assumed to have been loaded
+// from the given filename) and returns the zcl.File object representing it.
+func (p *Parser) ParseJSON(src []byte, filename string) (*zcl.File, zcl.Diagnostics) {
+	if existing := p.files[filename]; existing != nil {
+		return existing, nil
+	}
+
+	file, diags := json.Parse(src, filename)
+	p.files[filename] = file
+	return file, diags
+}
+
+// ParseJSONFile reads the given filename and parses it as JSON, similarly to
+// ParseJSON. An error diagnostic is returned if the given file cannot be read.
+func (p *Parser) ParseJSONFile(filename string) (*zcl.File, zcl.Diagnostics) {
+	if existing := p.files[filename]; existing != nil {
+		return existing, nil
+	}
+
+	file, diags := json.ParseFile(filename)
+	p.files[filename] = file
+	return file, diags
+}
+
+// AddFile allows a caller to record in a parser a file that was parsed some
+// other way, thus allowing it to be included in the registry of sources.
+func (p *Parser) AddFile(filename string, file *zcl.File) {
+	p.files[filename] = file
+}
+
+// Sources returns a map from filenames to the raw source code that was
+// read from them. This is intended to be used, for example, to print
+// diagnostics with contextual information.
+//
+// The arrays underlying the returned slices should not be modified.
+func (p *Parser) Sources() map[string][]byte {
+	ret := make(map[string][]byte)
+	for fn, f := range p.files {
+		ret[fn] = f.Bytes
+	}
+	return ret
+}