Enable libexec artifacts

This commit allows the user to specify artifacts to be installed under
`usr/libexec`. I used the following documentation as a guide:

https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html#:~:text=Purpose,subdirectory%20under%20%2Fusr%2Flibexec%20.

Below, you can see at a glance what the behavior of various settings
will be. The following examples assume we are building a spec with
top-level `name: docker`. The subpath field defaults to the package
name.

```yaml
 # goes to /usr/libexec/docker/docker-compose
libexec:
  bin/docker-compose:
    name: hello

 # goes to /usr/libexec/docker/cli-plugins/docker-compose
libexec:
  bin/docker-compose:
    subPath: docker/cli-plugins

 # goes to /usr/libexec/something_else/cli-plugins/docker-compose
libexec:
  bin/docker-compose:
    subPath: something_else/cli-plugins

 # goes to /usr/libexec/docker/cli-plugins/hello
libexec:
  bin/docker-compose:
    subPath: docker/cli-plugins
    name: hello

 # goes to /usr/libexec/docker/hello
libexec:
  bin/docker-compose:
    name: hello

```

Signed-off-by: Peter Engelbert <pmengelbert@gmail.com>

artifacts.go

@@ -12,6 +12,8 @@ import (
 type Artifacts struct {
 	// Binaries is the list of binaries to include in the package.
 	Binaries map[string]ArtifactConfig `yaml:"binaries,omitempty" json:"binaries,omitempty"`
+	// Libexec is the list of additional binaries that may be invoked by the main package binary.
+	Libexec map[string]ArtifactConfig `yaml:"libexec,omitempty" json:"libexec,omitempty"`
 	// Manpages is the list of manpages to include in the package.
 	Manpages map[string]ArtifactConfig `yaml:"manpages,omitempty" json:"manpages,omitempty"`
 	// DataDirs is a list of read-only architecture-independent data files, to be placed in /usr/share/

frontend/rpm/template.go

@@ -499,6 +499,14 @@ func (w *specWrapper) Install() fmt.Stringer {
 		}
 	}
 
+	if w.Spec.Artifacts.Libexec != nil {
+		libexecFileKeys := dalec.SortMapKeys(w.Spec.Artifacts.Libexec)
+		for _, k := range libexecFileKeys {
+			le := w.Spec.Artifacts.Libexec[k]
+			copyArtifact(`%{buildroot}/%{_libexecdir}`, k, &le)
+		}
+	}
+
 	configKeys := dalec.SortMapKeys(w.Spec.Artifacts.ConfigFiles)
 	for _, c := range configKeys {
 		cfg := w.Spec.Artifacts.ConfigFiles[c]

load.go

@@ -445,6 +445,13 @@ func (s *Spec) FillDefaults() {
 			s.Patches[k][i].Strip = &strip
 		}
 	}
+
+	for k, ac := range s.Artifacts.Libexec {
+		if s.Artifacts.Libexec[k].SubPath == "" {
+			ac.SubPath = s.Name
+			s.Artifacts.Libexec[k] = ac
+		}
+	}
 }
 
 func (s Spec) Validate() error {