JARS not found by MANIFEST.FM

Question

I'm currently developing an eclipse plug-in, so I just started using JDE. I referrenced all the JARs needed in my project, yet I keep having errors in my MANIFEST.MF :

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Module-builder
Bundle-SymbolicName: module-builder;singleton:=true
Bundle-Version: 1.0.0.qualifier
Bundle-Activator: module_builder.Activator
Require-Bundle: org.eclipse.ui,
 org.eclipse.core.runtime
Bundle-RequiredExecutionEnvironment: JavaSE-1.7
Bundle-ActivationPolicy: lazy

the errors being

org.eclipse.ui cannot be resolved
org.eclipse.core.runtime cannot be resolved

Where could those errors be coming from ?

EDIT : As asked, the .project content :

 <?xml version="1.0" encoding="UTF-8"?>
<projectDescription>
        <name>module-builder</name>
        <comment></comment>
        <projects>
        </projects>
        <buildSpec>
                <buildCommand>
                        <name>org.eclipse.jdt.core.javabuilder</name>
                        <arguments>
                        </arguments>
                </buildCommand>
                <buildCommand>
                        <name>org.eclipse.pde.ManifestBuilder</name>
                        <arguments>
                        </arguments>
                </buildCommand>
                <buildCommand>
                        <name>org.eclipse.pde.SchemaBuilder</name>
                        <arguments>
                        </arguments>
                </buildCommand>
        </buildSpec>
        <natures>
                <nature>org.eclipse.pde.PluginNature</nature>
                <nature>org.eclipse.jdt.core.javanature</nature>
        </natures>
</projectDescription>