vpm: support `v install -once module_name`(#13977)

cmd/tools/vpm.v

@@ -87,6 +87,12 @@ fn main() {
 				module_names = manifest.dependencies
 			}
 			mut source := Source.vpm
+			if '--once' in options {
+				module_names = vpm_once_filter(module_names)
+				if module_names.len == 0 {
+					return
+				}
+			}
 			if '--git' in options {
 				source = Source.git
 			}
@@ -327,6 +333,17 @@ fn vpm_install_from_vcs(module_names []string, vcs_key string) {
 	}
 }
 
+fn vpm_once_filter(module_names []string) []string {
+	installed_modules := get_installed_modules()
+	mut toinstall := []string{}
+	for mn in module_names {
+		if mn !in installed_modules {
+			toinstall << mn
+		}
+	}
+	return toinstall
+}
+
 fn vpm_install(module_names []string, source Source) {
 	if settings.is_help {
 		vhelp.show_topic('install')
@@ -336,17 +353,18 @@ fn vpm_install(module_names []string, source Source) {
 		println('´v install´ requires *at least one* module name.')
 		exit(2)
 	}
-
-	if source == .vpm {
+	match source {
+		.vpm {
 			vpm_install_from_vpm(module_names)
 		}
-	if source == .git {
+		.git {
 			vpm_install_from_vcs(module_names, 'git')
 		}
-	if source == .hg {
+		.hg {
 			vpm_install_from_vcs(module_names, 'hg')
 		}
 	}
+}
 
 fn vpm_update(m []string) {
 	mut module_names := m.clone()

cmd/v/help/install.txt

@@ -8,6 +8,7 @@ Options:
   --vpm         - [Default] Install from vpm
   --git         - Install from git repository url
   --hg          - Install from mercurial repository url
+  --once        - Only install the module if it was not previously installed
   -help         - Show usage info.
   -v            - Print more details about the performed operation.
   -server-url   - When doing network operations, use this vpm server. Can be given multiple times.

doc/docs.md

@@ -4601,13 +4601,19 @@ v install ui
 
 Modules can be installed directly from git or mercurial repositories.
 ```powershell
-v install [--git|--hg] [url]
+v install [--once] [--git|--hg] [url]
 ```
 **Example:**
 ```powershell
 v install --git https://github.com/vlang/markdown
 ```
 
+Sometimes you may want to install the dependencies **ONLY** if those are not installed:
+
+```
+v install --once [module]
+```
+
 Removing a module with v:
 
 ```powershell