refactor: change default namespace from "mcpp" to "mcpplibs"

Bare dependency declarations (without explicit namespace subtable)
now default to the "mcpplibs" namespace instead of "mcpp". This
matches the actual package ecosystem where modular C++23 libraries
live under the mcpplibs namespace.

Effect: `[dependencies] cmdline = "0.0.2"` is now equivalent to
`[dependencies.mcpplibs] cmdline = "0.0.2"`, eliminating the need
to always spell out the namespace subtable for the common case.

Non-modular (compat) packages still require explicit declaration:
`[dependencies.compat] mbedtls = "3.6.1"`.

Author: Sunrisepeak

src/pm/dep_spec.cppm

@@ -39,6 +39,6 @@ struct DependencySpec {
 
 // Default namespace for packages declared without an explicit one — the
 // mcpp-index "root". Bare `gtest = "1.15.2"` becomes `(mcpp, gtest)`.
-inline constexpr std::string_view kDefaultNamespace = "mcpp";
+inline constexpr std::string_view kDefaultNamespace = "mcpplibs";
 
 } // namespace mcpp::pm

tests/unit/test_manifest.cpp

@@ -207,11 +207,11 @@ foo   = { path = "../foo" }
     ASSERT_TRUE(m.has_value()) << m.error().format();
     ASSERT_EQ(m->dependencies.size(), 2u);
     auto& g = m->dependencies.at("gtest");
-    EXPECT_EQ(g.namespace_, "mcpp");
+    EXPECT_EQ(g.namespace_, "mcpplibs");
     EXPECT_EQ(g.shortName,  "gtest");
     EXPECT_EQ(g.version,    "1.15.2");
     auto& f = m->dependencies.at("foo");
-    EXPECT_EQ(f.namespace_, "mcpp");
+    EXPECT_EQ(f.namespace_, "mcpplibs");
     EXPECT_EQ(f.shortName,  "foo");
     EXPECT_EQ(f.path,       "../foo");
 }
@@ -244,7 +244,7 @@ gtest = "1.15.2"
     EXPECT_EQ(tmpl.version,    "0.0.1");
 
     auto& gtest = m->dependencies.at("gtest");
-    EXPECT_EQ(gtest.namespace_, "mcpp");
+    EXPECT_EQ(gtest.namespace_, "mcpplibs");
     EXPECT_EQ(gtest.shortName,  "gtest");
     EXPECT_EQ(gtest.version,    "1.15.2");
 }
@@ -287,7 +287,7 @@ util = "2.0.0"
     ASSERT_TRUE(m.has_value()) << m.error().format();
     ASSERT_EQ(m->dependencies.size(), 2u);
     auto& bar = m->dependencies.at("bar");
-    EXPECT_EQ(bar.namespace_, "mcpp");
+    EXPECT_EQ(bar.namespace_, "mcpplibs");
     EXPECT_EQ(bar.shortName,  "bar");
     EXPECT_EQ(bar.git,        "https://example.com/bar.git");
     EXPECT_EQ(bar.gitRev,     "v1");
@@ -320,7 +320,7 @@ package = {
     ASSERT_EQ(m->dependencies.size(), 2u);
 
     auto& a = m->dependencies.at("mbedtls");
-    EXPECT_EQ(a.namespace_, "mcpp");
+    EXPECT_EQ(a.namespace_, "mcpplibs");
     EXPECT_EQ(a.shortName,  "mbedtls");
     EXPECT_EQ(a.version,    "3.6.1");