make Maven parent path relative on current project (#987)

Previously, the path is always relative to the provided path to
`pom.xml`, however, this path should be relative to the current parent
path.

Also we observe that sometimes `pom.xml` is omitted in `</relativePath>`
so when this happens, we manually append `pom.xml` to file path.

internal/resolution/manifest/fixtures/parent/grandparent/pom.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>org.grandparent</groupId>
+  <artifactId>grandparent-pom</artifactId>
+  <version>1.1.1</version>
+
+  <name>my-app</name>
+  <!-- FIXME change it to the project's website -->
+  <url>http://www.example.com</url>
+
+  <packaging>pom</packaging>
+
+  <parent>
+    <groupId>org.upstream</groupId>
+    <artifactId>parent-pom</artifactId>
+    <version>1.2.3</version>
+  </parent>
+
+</project>

internal/resolution/manifest/fixtures/parent/pom.xml

@@ -15,9 +15,10 @@
   <packaging>pom</packaging>
 
   <parent>
-    <groupId>org.upstream</groupId>
-    <artifactId>parent-pom</artifactId>
-    <version>1.2.3</version>
+    <groupId>org.grandparent</groupId>
+    <artifactId>grandparentparent-pom</artifactId>
+    <version>1.1.1</version>
+    <relativePath>./grandparent</relativePath>
   </parent>
 
   <properties>

internal/resolution/manifest/maven.go

@@ -187,6 +187,7 @@ func (m MavenManifestIO) Read(df lockfile.DepFile) (Manifest, error) {
 const MaxParent = 100
 
 func (m MavenManifestIO) MergeParents(ctx context.Context, result *maven.Project, current maven.Parent, start int, path string, addRequirements func(maven.Project, string), prefix string) error {
+	currentPath := path
 	visited := make(map[maven.ProjectKey]bool, MaxParent)
 	for n := start; n < MaxParent; n++ {
 		if current.GroupID == "" || current.ArtifactID == "" || current.Version == "" {
@@ -200,7 +201,12 @@ func (m MavenManifestIO) MergeParents(ctx context.Context, result *maven.Project
 
 		var proj maven.Project
 		if current.RelativePath != "" {
-			f, err := os.Open(filepath.Join(filepath.Dir(path), string(current.RelativePath)))
+			currentPath = filepath.Join(filepath.Dir(currentPath), string(current.RelativePath))
+			if filepath.Base(currentPath) != "pom.xml" {
+				// If the base is not pom.xml, this path is a directory but not a file.
+				currentPath = filepath.Join(currentPath, "pom.xml")
+			}
+			f, err := os.Open(currentPath)
 			if err != nil {
 				return fmt.Errorf("failed to open parent file %s: %w", current.RelativePath, err)
 			}