Fix -imports handling aliased imports in source mode (#165)

In source mode, when generating a mock within the same package that uses
names from a package that is imported with an alias, the generated mock
should obey -imports properly and generate imports with the provided
alias.

If the generated code is not using the same aliases as the main filed in
a package the go compiler will still happily run but if you want ot use
this package as a source in another package where you want to generate a
mock, mock will fail because it will not know what to do with a package
that is imported with two different names in a source package.

This patch fixes the generation by making `-imports` handling do a more
correct thing.

It can be argued that the correct behavior is, by default use the the
same package aliases as the ones in the source file, however that change
looked way more invasive and i didn't see a good coverage of tests that
would help me make sure the changes worked well.

This is what i found to be the least invasive fix for #166.

mockgen/internal/tests/import_aliased/source.go

@@ -0,0 +1,11 @@
+package import_aliased
+
+import (
+	definitionAlias "context"
+)
+
+//go:generate mockgen -package import_aliased -destination source_mock.go -source=source.go -imports definitionAlias=context
+
+type S interface {
+	M(ctx definitionAlias.Context)
+}

mockgen/mockgen.go

@@ -393,8 +393,8 @@ func (g *generator) Generate(pkg *model.Package, outputPkgName string, outputPac
 		// try base0, base1, ...
 		pkgName := base
 
-		if _, ok := definedImports[base]; ok {
-			pkgName = definedImports[base]
+		if _, ok := definedImports[pth]; ok {
+			pkgName = definedImports[pth]
 		}
 
 		i := 0