Fix broken doc links for packages without a version

The selected packages sidebar built each link as
`hexdocs_package_url(name) <> "/" <> ver <> "/"`, which produced a
double slash (e.g. `mix.hexdocs.pm//`) whenever the resolved version was
empty, breaking the link.

Introduce `hexdocs_package_version_url/2`, which omits the version
segment when the version is empty and otherwise appends it without a
trailing slash.

Author: ericmj

src/hexdocs/config.gleam

@@ -17,6 +17,15 @@ pub fn hexdocs_package_url(package: String) -> String {
   "https://" <> hexdocs_package_host(package)
 }
 
+/// Documentation URL for a package at a specific version. When the version is
+/// empty the version segment is omitted, linking to the latest documentation.
+pub fn hexdocs_package_version_url(package: String, version: String) -> String {
+  case version {
+    "" -> hexdocs_package_url(package)
+    _ -> hexdocs_package_url(package) <> "/" <> version
+  }
+}
+
 pub fn hexpm_url() -> String {
   "https://hex.pm"
 }

src/hexdocs/view/search.gleam

@@ -245,10 +245,10 @@ pub fn search(model: Model) {
                             case filter.status {
                               version.Found(ver) ->
                                 attribute.href(
-                                  config.hexdocs_package_url(filter.name)
-                                  <> "/"
-                                  <> ver
-                                  <> "/",
+                                  config.hexdocs_package_version_url(
+                                    filter.name,
+                                    ver,
+                                  ),
                                 )
                               _ -> class("")
                             },

test/hexdocs_test.gleam

@@ -1,12 +1,22 @@
 import gleeunit
 import gleeunit/should
+import hexdocs/config
 
 pub fn main() {
   gleeunit.main()
 }
 
-// gleeunit test functions end in `_test`
-pub fn hello_world_test() {
-  1
-  |> should.equal(1)
+pub fn hexdocs_package_version_url_with_version_test() {
+  config.hexdocs_package_version_url("phoenix", "1.7.0")
+  |> should.equal("https://phoenix.hexdocs.pm/1.7.0")
+}
+
+pub fn hexdocs_package_version_url_without_version_test() {
+  config.hexdocs_package_version_url("mix", "")
+  |> should.equal("https://mix.hexdocs.pm")
+}
+
+pub fn hexdocs_package_version_url_replaces_underscores_test() {
+  config.hexdocs_package_version_url("ex_unit", "")
+  |> should.equal("https://ex-unit.hexdocs.pm")
 }