Add test case for non-github hostname

Tests results on current branch; install succeeded which was not expected
```
2021/02/16 14:02:24 ui: Installed plugin example.com/sylviamoss/comment v0.2.19 in "/tmp/pkr-test-cfg-dir-6_pkr_config458005728/example.com/sylviamoss/comment/packer-plugin-comment_v0.2.19_x5.0_linux_amd64"
    init_test.go:361: InitCommand.Run() = 0, want 1
    init_test.go:381: unexpected dir hash after init:   string(
        -       "h1:47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=",
        +       "h1:iVtzkl/nVm2KiLvlz8rH56ME8QEqRxq8+XT2Lo6bzGU=",
          )
--- FAIL: TestInitCommand_Run (6.39s)
    --- PASS: TestInitCommand_Run/already-installed-no-op (0.01s)
        --- PASS: TestInitCommand_Run/already-installed-no-op/-subtest-0 (0.00s)
    --- PASS: TestInitCommand_Run/already-installed-upgrade (2.30s)
        --- PASS: TestInitCommand_Run/already-installed-upgrade/-subtest-0 (0.06s)
    --- PASS: TestInitCommand_Run/release-with-no-binary (0.17s)
    --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-works (1.32s)
        --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-works/-subtest-0 (0.01s)
    --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-old-api-fails (1.42s)
        --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-old-api-fails/-subtest-0 (0.01s)
    --- FAIL: TestInitCommand_Run/unsupported-non-github-source-address (1.18s)
```

Tests results after change with change in this branch
```
2021/02/16 14:03:14 [TRACE] getting available versions for the example.com/sylviamoss/comment plugin
2021/02/16 14:03:14 [TRACE] &{%!q(*github.Client=<nil>) "packer-getter-github-1.7.0-dev"} getter could not get release: example.com/sylviamoss/comment doesn't appear to be a valid github.com source address; check source and try again.
2021/02/16 14:03:14 [DEBUG] will try to install: []
2021/02/16 14:03:14 ui error: no release version found for the example.com/sylviamoss/comment plugin matching the constraint(s): "v0.2.19"
--- PASS: TestInitCommand_Run (5.38s)
    --- PASS: TestInitCommand_Run/already-installed-no-op (0.01s)
        --- PASS: TestInitCommand_Run/already-installed-no-op/-subtest-0 (0.00s)
    --- PASS: TestInitCommand_Run/already-installed-upgrade (2.08s)
        --- PASS: TestInitCommand_Run/already-installed-upgrade/-subtest-0 (0.07s)
    --- PASS: TestInitCommand_Run/release-with-no-binary (0.21s)
    --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-works (1.20s)
        --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-works/-subtest-0 (0.01s)
    --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-old-api-fails (1.88s)
        --- PASS: TestInitCommand_Run/manually-installed-single-component-plugin-old-api-fails/-subtest-0 (0.01s)
    --- PASS: TestInitCommand_Run/unsupported-non-github-source-address (0.00s)
```

command/init_test.go

@@ -291,6 +291,32 @@ func TestInitCommand_Run(t *testing.T) {
 				testBuild{want: 1}.fn,
 			},
 		},
+		{
+			"unsupported-non-github-source-address",
+			[]func(t *testing.T, tc testCaseInit){
+				skipInitTestUnlessEnVar(acctest.TestEnvVar).fn,
+			},
+			testMetaFile(t),
+			nil,
+			"h1:47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=",
+			map[string]string{
+				`cfg.pkr.hcl`: `
+					packer {
+						required_plugins {
+							comment = {
+								source  = "example.com/sylviamoss/comment"
+								version = "v0.2.19"
+							}
+						}
+					}`,
+			},
+			cfg.dir("6_pkr_config"),
+			cfg.dir("6_pkr_user_folder"),
+			1,
+			nil,
+			"h1:47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=",
+			nil,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {