tools: require the presence of a v.mod file, to install external urls via vpm (#19825)

cmd/tools/vpm/common.v

@@ -56,6 +56,10 @@ fn parse_query(query []string) ([]Module, []Module) {
 				errors++
 				continue
 			}
+			if !has_vmod(ident) {
+				errors++
+				continue
+			}
 			Module{
 				name: name
 				url: ident
@@ -93,6 +97,25 @@ fn parse_query(query []string) ([]Module, []Module) {
 	return vpm_modules, extended_modules
 }
 
+fn has_vmod(url string) bool {
+	head_branch := os.execute_opt('git ls-remote --symref ${url} HEAD') or {
+		vpm_error('failed to find git HEAD for `${url}`.', details: err.msg())
+		return false
+	}.output.all_after_last('/').all_before(' ').all_before('\t')
+	url_ := if url.ends_with('.git') { url.replace('.git', '') } else { url }
+	manifest_url := '${url_}/blob/${head_branch}/v.mod'
+	vpm_log(@FILE_LINE, @FN, 'manifest_url: ${manifest_url}')
+	has_vmod := http.head(manifest_url) or {
+		vpm_error('failed to retrieve module data for `${url}`.')
+		return false
+	}.status_code == 200
+	if !has_vmod {
+		vpm_error('failed to find `v.mod` for `${url}`.')
+		return false
+	}
+	return true
+}
+
 fn get_mod_date_info(mut pp pool.PoolProcessor, idx int, wid int) &ModuleDateInfo {
 	mut result := &ModuleDateInfo{
 		name: pp.get_item[string](idx)

cmd/tools/vpm/install_test.v

@@ -1,5 +1,7 @@
 // vtest flaky: true
 // vtest retry: 3
+module main
+
 import os
 import v.vmod
 
@@ -108,3 +110,12 @@ fn test_missing_repo_name_in_url() {
 	assert res.exit_code == 1
 	assert res.output.contains('failed to retrieve module name for `${incomplete_url}`')
 }
+
+fn test_missing_vmod_in_url() {
+	assert has_vmod('https://github.com/vlang/v') // head branch == `master`.
+	assert has_vmod('https://github.com/v-analyzer/v-analyzer') // head branch == `main`.
+	assert !has_vmod('https://github.com/octocat/octocat.github.io') // not a V module.
+	res := os.execute('${v} install https://github.com/octocat/octocat.github.io')
+	assert res.exit_code == 1
+	assert res.output.contains('failed to find `v.mod` for `https://github.com/octocat/octocat.github.io`'), res.output
+}