resolveCommonAgreementFilePath (configKey, {
    defaultValue,
    siteAgreement,
    themeAgreement
  }) {
    const siteConfigValue = this.siteConfig[configKey]
    siteAgreement = this.resolveSiteAgreementFile(siteAgreement)

    const themeConfigValue = this.getThemeConfigValue(configKey)
    themeAgreement = this.resolveThemeAgreementFile(themeAgreement)

    return fsExistsFallback([
      siteConfigValue,
      siteAgreement,
      themeConfigValue,
      themeAgreement,
      defaultValue
    ].map(v => v))
  }
Resolve a path-type config.

@param {string} configKey
@param {string} defaultValue an absolute path
@param {string} siteAgreement a relative path to vuepress dir
@param {string} themeAgreement a relative path to theme dir
@returns {string | void}